[LeMans335d]

Today I noticed a subtle whooshing sound when I let off the accelerator after a hard acceleration. I've had the car for three years and I've never heard that noise before. (I'm pretty in-tune with the normal sounds my car makes) Anyone had a similar issue with an unmodified car?

Also, I hadn't drove the car for a few weeks and when I started it yesterday there was a slight electrical burning smell from under the hood but I couldn't track it down, not sure if the two issues are related. No CEL and no soft codes either when I checked.

Car is a 2011 335d with 93k miles. Only mods are a larger intercooler and swirl flap delete, both were done by me during CBU cleaning and vacuum hose replacement a year ago.

[BB_cuda]

Two answers
1) Our diesels don’t have a blow off valve so what you’re hearing is backwards airflow from letting off. The turbo has just created high P before letting off and then when you back off the high P pushes air backwards.
My car has the ATM cool air inlet at the noise is more pronounced as a turbo bark. It sounds like a tire squeal sort of. The stock air filter holder mutes lots of those noises. In BMW speak, it’s called an air silencer.
2) you should check your exhaust pressure sensor that has a down stream rubber hose. It can get partially burnt through and then big trouble starts. Fire will shoot out the hole and burn your engine harness. Exhaust P sensor is screwed into the cast iron exhaust manifold. I’m not home now or I post a picture. Good luck finding the burnt smell under hood.

[lnxguy]

Electrical burning smell might be your harmonic balancer. Although you wouldn't have gotten far if this was the case.

That noise you're hearing is compressor surge. Nothing to really worry about, happens in even stock cars (But the stock intake is good as quieting things down)