2002 BMW 325

Hi, I sincerely hope you can help to identify the issue I am having. I am hearing a subtle wheezing or "woooing" noise only when the vehicle is at around 800-1250rpm, both accelerating and decelerating. Over 1250rpm the noise just disappear (it could have been masked by the regular engine sound but at least at 1500rpm the car was very quiet, no whooing). It's more noticeable especially when the vehicle is decelerating. I have changed numerous parts hoping to fix the problem with no success. Below is what I have replaced,

- Lower and upper intake boot
- Tensioner pulley
- Idler pulley
- Alternator pulley
- AC belt
- Serpentine belt
- alternator
- DISA removed and checked
- Fan clutch (found to have play)
- Power steering fluid level topped up

This problem was first noticed about 3 months ago after my mechanic topped up the freon. I switched the A/C and even the air circulation off and the noise still can be heard, but not as noticeable as when A/C is on. It tells me it's not the AC. The AC probably makes it a little louder at times but it's not the root cause.

I brought the vehicle to a couple reputable local Bimmer shops and the mechanics told me the noise comes from when the alternator recharges and it's normal. I am not convinced because I have driven other 325xi without the noise. Any feedback would be appreciated. Also, if the noise is normal I wouldn't think anyone would have bought a bimmer.

I have done a lot of research but seems to be hitting the wall. Please, I love my bimmer and I hope you knowledgeable folks can help.