Unofficialbmw.com

[edjack]

Have you tired to vary the HVAC fan speed to see if it causes the whistle to vary? Maybe there's a leak in the ducting inder the hood.

[charliesmith]

Sham,

Your problem reminds me of another problem I encountered 23+ years ago on a '75 Volvo 245 wagon. The Dr. that owned it said it had developed a mysterious whistle coming from the front of the car. It only varied in intensity with the speed. After all the "old salt experts" in the mechanic shop gave up on locating the source, the car was sent to the body shop. I, being the jr. employee got shouldered with the task. The Dr. was an excellent customer with the dealership and the Service Manager told me to work only on that car until the problem was solved. It drove me nuts. Two days later (no lie) I located the source of the whistle. The whistle was coming from the thin aluminum trim strip that goes all the way across the front of the hood. There was a small gap between the trim strip and the hood, allowing air to blow under the strip and create a turbulence whistle. A rotating brush from a drive-through car wash had snagged the trim strip. Filling the strip cavity with silicone and reinstalling the strip solved the problem. I got so desperate that I had an assistant drive the car while I laid spread eagle on the hood, held on to the fenders and listened for the whistle source. A human hood ornament. It looked like Mel Gibson in a Lethal Weapon action scene. Those kind of problems you never forget. My reward? A two hour lunch break.

Your problem might be a variance of the above.

[edjack]

Here's another possibility: the fuel pressure regulator. I don't know where it resides in the engine compartment, but, especially at idle and low engine load, they will rattle or sing at pretty much the same pitch.

I don't seem to hear mine, so I can't definitely say this is it.