My engine makes what I can best describe as a whistling or beeping noise, which could be a squeaky belt of some sort. I noticed it over a year ago now and it hasn't caused me any problems so I have to assume it's normal, or at least not harmful. I'm not sure if it's the same sound you're hearing, but I made a thread about it with a video so you could check it out:
http://www.e90post.com/forums/showth...hlight=beeping