I have a 2004 325i, it has about 70,000 miles, and I am hearing the strangest noise out of the rear of the car. Its is a very high pitch and sounds rotational. The best way to describe it is like a mosquito's buzzing. The noise is always present after 5 - 10 minutes driving, when its dry and 30 - 45 minutes when the roads are wet. The pitch does not seem to change with the wheel bearings are loaded, when the brakes are applied or when the parking break is slightly applied. The noise gets louder as speed increases, but the pitch does not really fluctuate. It usually goes away under 5 mph, and is exceptionally loud when driving between 25 and 40 along barriers or fences (with the window down). Over 50 it becomes hard to hear as wind/engine/tire noise drowns it out.
I have checked the brakes, the rotors, the parking break shoes, the U-joints, the bearing seals, and the diff. When the car is up in the air and the wheels spinning, the noise is not present. I've been to 3 shops, and the only thing I was told was that the parking brake shoes were seated incorrectly, they ground them down, and put them back in. Supposedly curing the problem, BUT as I drove it home the noise reared its ugly head and continued to taunt me
Everyone who hears the high pitched whining seems to agree that it is not a serious problem, and I concur, but it is probably the most annoying noise I have every heard the car make.
If anyone has experienced this, or has any advice on what to check next, PLEASE jump in and let me know.