Engine sound fluctuation / oscillation question

Hey all, first time posting to a forum, so forgive me if this being posted in the wrong spot etc.

Ive noticed a strange '' oscillation '' in my engines noise as I'm driving. Finding it hard to describe.. So bare with me It only really happens between 2900ish to 3500ish rpm. The actual engine sounds normal, but its as if it gets loud then quiet, loud then quiet etc. It starts slow ish, but gets faster until about 3500 rpm when it fades and the engine sounds consistent again. I can feel a slight vibration in my gas pedal which is in sync with the oscillation. Doesn't seem to matter which gear I'm in. I should also note that it seems louder if I'm going up a hill or the engine is under more load.

The car is a 2002 Subaru legacy 2.5 N/A

Standard transmission

140k miles.

Ive just had the head gaskets replaced, fairly new spark plugs, pcv valve, fuel filter. (I noticed the noise when I got my car back from the head gasket repair, but it may have been there before and I didn't notice)

I have no clue what could be causing this, or maybe it's just the way these boxer engines sound?? This is my first subie. It doesn't sound "broken" haha. It feels solid and runs well.

Have you ever replaced the catalytic converter section on that car? If you look underneath the car its the part of the exhaust with two bulges after the Y shaped exhaust manifold.

If it looks newer than the rest of the exhaust it may have been replaced with a cheap unit, if it looks original it might be clogged/failing.

If this oscillation is associated to a vibration it could be something on the engine e.g. accessory drive pulley or something in the drivetrain e.g. diff, hub, gearbox bearing.

Have you ever replaced the catalytic converter section on that car? If you look underneath the car its the part of the exhaust with two bulges after the Y shaped exhaust manifold.

 

If it looks newer than the rest of the exhaust it may have been replaced with a cheap unit, if it looks original it might be clogged/failing.

 

If this oscillation is associated to a vibration it could be something on the engine e.g. accessory drive pulley or something in the drivetrain e.g. diff, hub, gearbox bearing.

The first cat after the manifold does have a rattle.. I've been meaning to take a look at it. Maybe it's on its way out, I've noticed a faint rotten egg smell once or twice in the past couple months. Ill look into that, thanks! Wouldn't have guessed that could be the culprit. Theres no vibration or other sounds associated with the oscillation.

If you have the smell its prob the cat! that smell is what the catalyst substrate smells like as is burns.

If looking for a replacement go with OEM, BOSAL or WALKER.

If you have the smell its prob the cat! that smell is what the catalyst substrate smells like as is burns.

 

If looking for a replacement go with OEM, BOSAL or WALKER.

Good to know, thanks! One weird thing, though.. I did a sea foam treatment the other day (spray into intake, poured some into gas tank) and the oscillation seems to be much less noticeable.. Could the sea foam have loosened some carbon and clogged the cat even more making it vibrate less somehow?