engine rattle at 2500-ish rpm... thoughts?

So the past week I have noticed the engine sounds a more pronounced rattle at around 2500 rpm - it seems to go away above 3000rpm, but it could just be getting drowned out by engine noise. My thoughts immediately go to "is this rod knock" but I can't see why it would be - engine oil is fine (I run castrol 0w40 synthetic) and the car drives just fine - no other symptoms. I like to drive quickly, but I don't race or really rev the car too high, anyway (not someone to constantly bump off the rev limiter or anything like that - it's my daily driver and generally takes me to and from work)

 

The other possibility I think would be the timing belt tensioner, as they can make a similar knocking sound. I can't find anything loose like the serpentine belt tensioner or any heat shields (not in the rust belt, so corrosion isn't really an issue)

 

All of the rod knock sounds I have heard are much louder and more pronounced than this... they sound more like a clacking sound, where this is a little more like a ticking sound. I don't hear it at all until above 2100-2500 rpm, then it really goes away above 3000rpm

 

The car really hasn't ever thrown codes - the only time, and maybe this is relevant - was a few weeks ago en route to work, when the brake light started flashing and the CEL started flashing - everything seemed normal, but I immediately pulled over, shut it off, and all was fine - when I pulled codes that night it had misfire codes for 2 cylinders, but it never came back - with the flashing brake light I figured a glitch)

I wouldn't eliminate any possibilities, but start with the more obvious things like loose heat shields first.

Oooofa. Mine started knocking/rattling this week around 2,500 rpm. Only when under no load. I too was hoping for the tensioner, but dealer said that is fine. They are pulling oil pan Monday to have a look-see.

Mine seems to be the same if I am driving under load, or just revving in neutral. The dealership is open tomorrow and I was asked to bring it in at 8am, so we'll see what they say... I changed the oil and there might have been some debris in the drain pain, but not a lot, and I have recently cut some things up that I had over the oil pan, so it could be residual debris from that... I wasn't careful enough. Ran the oil through a sheet and didn't catch anything. I might have found some larger bits in the drain pan, but I was a moron and didn't clean it first... I run Castrol 0W40 synthetic, and there were only 1600 miles on the oil change... I probably don't even want to know what repairing this is going to cost if it's a spun bearing...

 

When I spoke with the service folks, they had the usual "does it do it hot/cold, etc." sort of questions - mine is insensitive to temperature - they did say that generally speaking, you can't rev-out a spun bearing/rod knock - so for it to go away at 3k rpm is a possibly good sign... my car only has 58k miles on it...

Piston slap is also a possibility. Hoping for the best for both of you. I would imagine rod knock would show up on the AP if you have it on the gauges. One more reason I leave mine in the car and attached, I like to see any knock events as they happen.

 

I do sometimes get a deep ticking sound, not quite a knock or a tapping but when I roll the window down to listen I can't quite place it, and it is somewhat rpm dependent. Usually in hotter weather though.

 

Rod knock would definitely be more pronounced when the engine is cold and most of the oil has drained, and then get worse again in the extreme hot.

Piston slap is usually temperature dependent, or at least should be when it's first developing. My Baja clatters like a cold Caterpillar when I start it after it sits outside, but the slap is all but gone when everything gets up to temp.

 

Trying to think of what else it could be- maybe the rubber core of the crank pulley is falling apart, and the outer chunk rattles at certain speeds?

Only making the noise around a specific RPM sounds like something is resonating to me. Like a vibration. It's hard to assist with diagnosis without hearing it in person though.

 

I would be very wary of a dealer's suggestions with this type of thing. Sometimes they like to use a big repair as a catch-all for something they can't figure out. If they tell you it's and engine mount or you need to replace your ECU it would be a big red flag.

Piston slap is also a possibility. Hoping for the best for both of you. I would imagine rod knock would show up on the AP if you have it on the gauges. One more reason I leave mine in the car and attached, I like to see any knock events as they happen.

 

I do sometimes get a deep ticking sound, not quite a knock or a tapping but when I roll the window down to listen I can't quite place it, and it is somewhat rpm dependent. Usually in hotter weather though.

 

Rod knock would definitely be more pronounced when the engine is cold and most of the oil has drained, and then get worse again in the extreme hot.

So it doesn't sound like piston slap, and isn't temperature dependent. The oil was maybe a cm or two at most off of full when I drained it, and is full now.

Only making the noise around a specific RPM sounds like something is resonating to me. Like a vibration. It's hard to assist with diagnosis without hearing it in person though.

 

I would be very wary of a dealer's suggestions with this type of thing. Sometimes they like to use a big repair as a catch-all for something they can't figure out. If they tell you it's and engine mount or you need to replace your ECU it would be a big red flag.

I was thinking it was a resonance as well - thinking maybe it was the timing belt tensioner loosening up a bit, and in a certain rpm range it acts up... I just don't know. I don't know how to diagnose rod knock other than to work to rule out everything else. I figured I'd give the dealer a shot at identifying what's acting up and see what they recommend.

 

Every time I have seen/heard a vehicle with rod knock it's really sharp sounding and you can hear it immediately as you increase the rpm, and more often than not, it shows up at idle... I don't know if that's what it turns into eventually, or if that's what happens right after the bearing is damaged.

With a stethoscope you could pinpoint the noise being in the block area or the timing front by the timing cover. The damper is an interesting thought too.

 

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I'd think it's a heatshield resonating as mentioned. Might help to get a video.

This is much more of a rattle/click/tick than the buzzing sort of sound you get with a heat shield. the speed of the clicking increases with engine speed, as well. It's at the dealer now, so I should know something later today. really, really, really hoping it's not a spun bearing...

Does it make any noise at start up?

 

My wrist pin/piston slap noise was around 2500RPM and at start up for a second or two.

Well, still somewhat indeterminate... they said it does not appear to be a connecting rod, and they think it could be the tensioner, but they won't be able to take the timing cover off and get in there until Monday or possibly Tuesday. On a positive note, they are giving me a loaner, so that's good!

Ok - so driving a 19 Legacy sport... they said they are almost certain it's not a connecting rod... I guess I'll believe it when I have the car back and it's not ticking any more...

Oooofa. Mine started knocking/rattling this week around 2,500 rpm. Only when under no load. I too was hoping for the tensioner, but dealer said that is fine. They are pulling oil pan Monday to have a look-see.

How did they confirm the tensioner was fine? you need to pull the crank pulley and timing cover to get to it. For my rattle they are confident it's not internal for some reason (stethoscope on the block, I imagine) but they didn't have time to pull the timing stuff today.

so if a connecting rod bearing is messed up, would you expect a log to show knocking events? or can it distinguish detonation from a bad bearing?

How did they confirm the tensioner was fine? you need to pull the crank pulley and timing cover to get to it. For my rattle they are confident it's not internal for some reason (stethoscope on the block, I imagine) but they didn't have time to pull the timing stuff today.

 

Damn good question. I had assumed they could pull the driver side portion of the timing cover and get close enough to the tensioner to hear it. I intend to stop in and chat with the tech Monday.

 

https://parts.annapolissubaru.com/a/Subaru_2011_Legacy-25L-TURBO-6MT-4WD-GT-Limited-Sedan/_54102_6030233/TIMING-BELT-COVER-10MY-12MY/B14-022-02.html

so if a connecting rod bearing is messed up, would you expect a log to show knocking events? or can it distinguish detonation from a bad bearing?

 

I would assume so, but it may be a different frequency that is not within the parameters of the stock knock sensors data range. But I have also seen many posts where the car is pulling timing with a low DAM when there is audible knocking present. It would be nice to see the raw data vs the data the ecm sees from the sensor to understand what its looking for.

Damn good question. I had assumed they could pull the driver side portion of the timing cover and get close enough to the tensioner to hear it. I intend to stop in and chat with the tech Monday.

 

https://parts.annapolissubaru.com/a/Subaru_2011_Legacy-25L-TURBO-6MT-4WD-GT-Limited-Sedan/_54102_6030233/TIMING-BELT-COVER-10MY-12MY/B14-022-02.html

Unfortunately, you can not - the tensioner is under the center timing cover, so to lay eyes on it, you need to remove the crank pulley and the center timing cover.

https://parts.annapolissubaru.com/a/Subaru_2011_Legacy-25L-TURBO-6MT-4WD-GT-Limited-Sedan/_54102_6022813/CAMSHAFT--TIMING-BELT-TIMING-BELT-10MY-12MY/B14-013-04.html

I would assume so, but it may be a different frequency that is not within the parameters of the stock knock sensors data range. But I have also seen many posts where the car is pulling timing with a low DAM when there is audible knocking present. It would be nice to see the raw data vs the data the ecm sees from the sensor to understand what its looking for.

On cars I have seen videos of where it's a really loud knocking/clacking, I wouldn't be surprised if the engine interpreted some of that as knock - though I would think a detonation event would look different than a clunk originating from the crankshaft. The raw data from the knock sensor would definitely be interesting to see - I guess the knock sensor is just a piezoelectric sensor, and if the ECM is just looking for a peak voltage crossing a threshold value, rather than a specific waveform, then maybe anything that gives enough of a mechanical shock to generate the voltage would make the engine think it's knocking...

Predetonation would cause the piston to knock inside the cylinder wall, the noise should be similar to a failing/failed bearing. The one subaru I drove with a really bad knock, the sound was very sharp, basically like hitting an anvil with a hammer.

I could keep my car from registering knock on my AP if I kept the knock quiet enough. It would knock all the time towards the end and if I stayed out of boost, kept the RPM's way low (read as barely drove the car and shifted around 2500k) it would't register knock/fine knock. I could get the DAM back to 1.00 if I was real careful too, but ultimately I would get into boost or need an upper RPM and the DAM would drop again.

 

The knock sensor seems pretty sensitive, but doesn't catch all noises under the hood.

I could keep my car from registering knock on my AP if I kept the knock quiet enough. It would knock all the time towards the end and if I stayed out of boost, kept the RPM's way low (read as barely drove the car and shifted around 2500k) it would't register knock/fine knock. I could get the DAM back to 1.00 if I was real careful too, but ultimately I would get into boost or need an upper RPM and the DAM would drop again.

 

The knock sensor seems pretty sensitive, but doesn't catch all noises under the hood.

 

I would say its more likely the programming ignores some noises and reacts to others, probably a predetermined frequency range, duration, rhythm etc. That would be my best guess. Maybe using some kind of scope on the sensor could show what it will and will not pick up, I would bet it can pickup a lot of different noises, but ,most of them are useless to the ecm.

and it's a spun bearing. They pulled the timing kit on tuesday and everything was fine - dropped the oil pan and it's pretty much full of bearing bits (they sent me a picture). Not sure how this even happens on a car I maintain the crap out of and pretty much just takes me to and from work. I only have 58k miles on it. And for anyone thinking the Subaru class action suit for the 12-17 WRX and STi which have the EJ25 in it, it does not (and while some websites indicate that settlement is through, apparently it has not been completed yet). Freaking sucks.