Q&A

Its so hard to guess at the source of a noise without being able to hear it first hand. I would start by checking the condition and tightness of the fan belts/drive belts at the front of the engine. A loose fan belt may squeal when the alternator is under load, the air conditioning or steering is operated.
Vacuum leaks can cause a intermittent squeal but would disappear when the car is accelerated under load as engine vacuum decreases on acceleration.

Sounds like what was happening to my 2001-325i; Turned out to be rear wheels--wheel bearings going bad. Had them changed (both were bad) at about $300.00 each. Good Luck!

sounds like you got a blow off valve.

i have a 330i and had the same problem. took a while to figure it out. its most likely coming from the lower intake boot. its a rubber elbow which is ripped from ribbed part of the tube. its going to cost u 28$ from bmw dealer ship and u can also find a video on youtube which shows u how to get it done

I had a similar sound coming from the engine bay and it turned out to be the D.I.S.A valve on the side of the intake manifold. The way you find out is have someone on the inside revving the motor if the sound is coming from intake area this is most likely your problem get this fix right away because it can cause catastrophic failure if the valve comes apart and gets sucked in to the intake... jope this helps