odd noise

Hey, my cars been making an odd noise from around 500-1500 RPM. it sounds almost as if the fans hitting something. It doesn't sound like anythings vibrating just a consitent tapping in that RPM range, any ideas what it may be?

Mine was the catalytic converter guts loose & rattling/buzzing inside the housing. I've also had a piece of gravel rattling between the catlytic converter and the heat shield.