Dropped iPhone 4 - rattling noise

I just dropped my iPhone 4 on the floor of my bedroom (had it in bed, fell asleep, flew off when I was getting up from bed). It's a hardwood floor, and I don't notice any cracks or dents or dings on the iPhone's external frame. I also don't seem to have any problems with calls, audio (headphone or external), or the camera. However, when I shake the phone near my ear I hear a slight rattle. Can't tell where it's coming from exactly, although it seems like it's around the middle/top of the phone. Anybody else had similar incidents, and if so do you know what this is? Is this something that I should go to the Apple store to get fixed? So far no functional problems, but the sound is a bit annoying. Thanks.