funny sound

This is my first post, ive been reading the forums for only about a week now and decided to ask for some help. I drive a 98 Civic LX and when i am at lower RPMs, under 3Kish i get a strange sound. The best i can describe it is like a playing card in a bike tire. It just kind of clicks over and over, but if i rev the engine, it goes away. Friend told me it might be a cracked header, but i have NO idea what it is. Coudl anybody help me out here?

Thanks in advance, and GREAT site.