I have a 2002 MCS with 92k. I recently installed a new CAI from M7 as a part of a series of engine upgrades. Before installation I never heard a whistling noise that I can only describe as a high pitch whistle kinda like feedback coming from an elderly persons hearing aids. The car has to be moving and the noise shows up between 2500 and 3500 RPM. It does not matter what gear or speed the car is doing or in. Though it is noticeable at highway speeds or if I feather the gas to keep RPM's near 3000. To me I'm thinking it's an air leak of some kind or another or a vacuum going bad. It is not the normal whine you hear coming from the supercharger because the whistling can be heard over the whine at times. I have no clue where to start looking and it's really starting to get annoying. Any help in figuring out this problem would help.

 

that noise is the throttle plate being open just a little bit (foot on the gas a little, or even at idle) the reason you didnt notice it with the stock air intake is because it acted as a baffle too. now all you have is a open filter. this is normal, so no worries

 

Yup...common...
You can hear it more with foam filters than with cotton ones, so if it is to annoying, try a filter change. This sound is present in most CAI (on the mini) to some degree, some almost none, some very noticable...some cars more than others too...

 

Uzzah--I agree with nate and zip. BTW: Our cars are very similar, except I'm using the K&N CIA. It's fairly quiet in my cabin, and I even have my firewall drilled for more cold air. Filter style does make a difference in noise.

 

Everyone, thanks for the advice. I kinda figured that since I opened up the airflow I was going to hear more engine noise. I had a friend who is also a performance mechanic say the same thing. Though he cautioned me that I may want to change the Foam Air Filter to a K&N type filter since the foam breaks down quicker and can cause some intake damage.