Originally Posted by
FBinNY
Could be as simple as a shaving or burr breaking off and rattling in the tubes. Did you remove the seat and see if you could shake something out?
If it is the water bottle bosses, solve it by putting a screw into any unused ones, and tightening.
Yeah, it doesn't quite sound like something moving freely within the frame, but sounds almost exactly like the washer on a presta valve when it's rattling against the wheel. At first I thought it was the chain because it rattles as long as the chain bounces when the bike is "dropped", but I've already ruled out the chain (and because I can hear it by tapping the frame...).