Sudden thick exhaust at idle?

This has happened twice now in the span of maybe 2 months (at least that I've noticed). While at idle for several seconds, my car suddenly billows out a LOT of white exhaust. The first time it was a fairly thick white, this second time it seemed more greyish, but I'm thinking it was just less dense. Each time I noticed this, I turned the car off for a couple seconds, and at re-start, there didn't seem to be visible exhaust.

I figured it was something burning off as there's no visible exhaust for several seconds, and then a giant cloud flies by me. I noticed some exhaust when I got on it on the freeway the other day but I believe that's normal?

I had 2 HC tests done about a year ago, one Subaru shop said it was positive for HC's in the coolant and the HG's were going, a second Subaru shop said negative, everything seemed fine. Since bad HG's are fairly uncommon in our cars, I figured I'd ride it out as it was not going through coolant and not over heating. A year later and nothings changed, except this exhaust thing.

I'm also dealing with the shuddering idle crap right now. Could these possibly be linked together somehow? I'm confused as this has only happened while sitting at idle for several seconds.

Is this smoke mainly on the first start up in the morning, if so that's normal.

Dose the car idle fine during this ?

Negative, both times it happened were after I had been driving the car on and off for hours and was in the afternoon. Not an external temp thing either as both times it was at least 55-60 degrees.

I believe the first time this happened it was idling ok, however the second time it was idling rough. The rough idle for me is happening at cold & normal operating temps, and is very on and off again so I'm not 100% sure.

White "smoke" is usually steam. Stand behind the car next time and sniff. If its caused by coolant you should be able to smell it right away. Oil burning is a bluish smoke and much different smell.

Sunds like a head gasket or head issue. Thats odd if it is intermittent though. Once the gasket goes it goes.

Generally speaking a puff of white smoke is not a major, its when you see blue smoke you should start worrying

It wasn't a puff though. It was like I had a mini fog machine in each exhaust for a couple seconds. Which would be cool if I was running onto the field at Husky stadium, but I wasn't.

White "smoke" is usually steam. Stand behind the car next time and sniff. If its caused by coolant you should be able to smell it right away. Oil burning is a bluish smoke and much different smell.

Sunds like a head gasket or head issue. Thats odd if it is intermittent though. Once the gasket goes it goes.

If it happens again I'll try, but it's hard to get out and go sniff my exhaust at a stop light in downtown Seattle, I don't want coffee thrown at me . FWIW I don't recall there being an odor the first time this happened.

Since it's not overheating at all, or going through coolant, I'm hoping its not a HG as that's a pretty penny I don't have atm. That's why I'm confused by the situation.