N18 engine noise after shut-off
 

I noticed a loud noise like something is spinning after shut-off, kind of a grinding noise. If I shut-off the engine at idle, I hear a really short noise. If I push the throttle fast to get to 3k then shut-off right away, I hear a really long drawn out sound like something is still spinning, but with a noise that is definitely not normal.

​​​​​​FYI, engine is a turbo. I recorded a video, which is only for the purpose of sound. In it, I start the car, blip the throttle, and shut-off the engine. You can then here this bad noise for a long time without the engine running.

Any ideas as to the source of the noise?

I only hear the engine spinning down from being revved up... unless i'm missing something

Yeah, I suppose that is a good description, but that noise never used to be noticable. It's really loud and has gotten worse over the last month.

It also doesn't do it all the time. For example, if I hold a steady 3k RPM and then shut-off, the long drawn out noise isn't there. If I blip the throttle really fast to 3k then shut down immediately, then the noise goes on like in the video.

The engine should be at idle when you shut it off. I fthe engine is at idle when shut-off, doe sit still sound like it runs on?

I always shut down the engine at idle, and recently I noticed a small grinding type noise going on for a split second longer than normal. I noticed this short sound getting louder. Then one day I accidentally shut-off the car with the accelerator pushed down just a bit and the sound went longer. I then tested shutting the car off with higher RPM and the sound went on for a long time.

I was wondering what could continue to spin for 10 to 15 seconds after the engine is shut-off and whatever that might be, could get louder as time went on. Only thing I can think of is the turbo. I got it inspected and the dealership didn't think it was the turbo. To my ears, it seems to come from the side of the engine facing the serpentine belt.

When you rev up and shut it off, you build pressure on the turbo and it spools up. As mentioned, do you hear it when you turn it off at idle speed?

If you suspect the serpentine belt area, take the belt off and recreate the noise. Does it come back?

So when the turbo spools up, would that be the only thing that would keep spinning after shutdown or is there anything else that could still be spinning to a stop, for 10 seconds afterwards (in the case of shutting down at high rpm).

But yes, there is a 1 second grinding noise that goes on after shutting down at idle. It almost sounds like what a bad bearing would sound like, but I don't know what that could possibly be in or on the engine.

The dealer told me it was a bad valve cover/pcv valve, and I just replaced that and it didn't fix the noise...

Sounds like the turbo bearing.
Turbo spins down after shut off.

I would almost say that your alternator is toast. I had a car in the shop making the same noise and it turned out that the alternator had huge amounts of play in the bearings.

Detension the belt and spin the alternator by hand, and even give the pulley a wiggle to see if there is any play in there, we've replaced a handful of alternators due to that failure.

But, there are some alternators that just have a bit of a "spool down" sound to them, even when the bearings feel fine and the alternator is working properly, that doesn't sound proper though...