I hate to be the guy with all the questions, but I guess somebody has to be.

Last night while driving home from work, I stopped at a light with my windows down. It became apparent that there was a rattle coming from the front of the car. I experimented on the rest of the drive, and noticed it went away when I increased the throttle, but still happened when stopped and at idle. I looked under the hood when I got back but didn't notice anything unusual, but it sounded like it was coming from below or behind the engine, not up top. Any thoughts guys? I'll be glad to give more info if you need.