metal rattling/clanging noise near exhaust manifold/headers?

After I drive my car for a while, sometimes I start to hear this sound, and it sounds like a metal piece is vibrating or clanging/rattling against some other piece of metal, and I put my ear under the hood and best I could tell it was coming from near the exhaust manifold/headers, i'm pretty sure that's where it was coming from. Anybody know what could be causing this noise? Is the heat shield loose or something?

get a stethiscope to make to find the were its coming from first

Your heat shield on the exhaust after the cat has rusted loose. I used a spring about 6 to 7 inches long with hooks on both ends and wrapped it around the head shield and exhaust. This stopped the rattling for good. The spring out lasted the exhaust. I got it from Home Depot for about $3.00 for two.