Whistling noise from hole in intake system

Thanks for all of the great advice and DIY tutorials, guys. I have accomplished more with your help on my BMW than I ever imagined.

A few months ago, I started getting a high-pitched whistling sound as I accelerated. I noticed a little bit of power loss and my gas mileage was also declining. Just a gut feeling, but I thought the whistling sound was air coming out of someplace it shouldn't be. I was going to replace the coolant expansion tank so thought I'd look for the cause of the whistling sound at the same time.

Here are two pictures of what I found, with the first pic taken while laying under the car and looking up at the expansion tank. I noticed a piece of plastic hanging down from what I think is part of the intake system. It was literally hanging on by just a few threads. I removed this part, pulled off the loose plastic piece, and wrapped the hole up with electrical tape. Figured that would help reduce air loss and keep dirt and debris out of my engine. Since covering this up with tape, power and mileage are better and the whistling noise is much quieter, however, it is still present. I want to replace this part asap but have no idea what it is. Can anyone give me a part name or number? It's for an '02 325i. Thanks again, in advance, for all your help.

it's an air channel to cool your alternator.
http://www.realoem.com/bmw/showparts...83&hg=12&fg=22

I think is here;
http://www.realoem.com/bmw/showparts...13&fg=20&hl=13

BVSJ, you nailed it! It is definitely the lower air channel. Good lookin out!