[Meanshyguy]

Ok so I got this car 2 years ago with 42k miles on it, it's a 09 335i coupe, the rear driver side muffler would always rattle a little since I got the car but would quiet down when the car warmed up a little so I thought nothing too bad of it.
I was at my aunts the other day since I noticed the rattling had gotten worse, I got behind the car and told my aunt to give it some gas, so hardly any fumes would come out through that muffler, the driver side was fine, yesterday I took it for a drive and I saw black smoke coming out while I was at a stop light.
Anybody here with some mechanical experience knows what could be causing this? This is my first turbo car so I'm afraid I fried a turbo building back pressure because of the clogged muffler. There are no check engine lights on dash, abnormal driving or poor performance.

[PA66400]

The driver's side exhaust has a valve that keeps it normally closed (no exhaust output) at idle when the engine is warm and at low revs. That part may be normal, but black smoke is not - it indicates a rich condition. If you have a rough idle, you may have an entirely unrelated problem with your direct injection. The slight rattle at idle may be the valve closing - it does rattle a little (not a lot) normally.

[Meanshyguy]

The idle is fine, it idles not over 1k when it's cold and drops to normal and quiets down, what scared me was the black smoke when I was at the red lights, any idea what it could be.

[e90pilot]

I was thinking it may have been a bit of soot buildup because your muffler was clogged but that should get worse when you give it more gas as you blow out the soot. It should go away once you blow it all out. It also happens if you haven't floored it in a while.

Does it smell at all? Maybe like raw gas? That would confirm PA66400's post that you might be running rich. You'd get an SES light for that though. Also look under there and make sure nothing's on fire or burning. That would make black smoke too.

[Meanshyguy]

Thanks I'll look into that when I go back home, it's my first turbo car so I thought the turbos might have been bad or something, about that muffler, it rattles pretty bad, everybody tells me if there's something wrong with it, so hopefully what e90pilot said it's the reason why it's smoking.

[e90pilot]

It could be waste gate rattle. I've never heard it in person so I can't really say. However I don't think that would cause black smoke.

[Meanshyguy]

I'm gonna put some aftermarket mufflers, hopefully that'll take care of the rattling at least, oh well, I guess I should've gotten rid of it earlier on.