I'm guess it as the water bottle cage too. My sworks Roubaix does this every now and again. I have to tighten the bolts holding the cage three of four time a year to keep it from sounding like a wheat thresher.
__________________
A conclusion is the place where you got tired of thinking. - S. Wright
Favorite rides in the stable: Indy Fab CJ Ti - Colnago MXL - S-Works Roubaix - Habanero Team Issue - Jamis Eclipse carbon/831