Car makes clanking sound from exhaust

After I run my car for any amount of time. My car will have a loud clanking clunking sound occasionally from the exhaust at the engine. sounds like something is like getting flicked against the inner wall or something. I listen from underneath the car, sound doesn't originate from under, or near the back. I wonder if it can be a bad catalytic convert, but am not too sure what it is. Car runs great. no leaks, no problems anymore. It make this sound prior to me rebuilding my engine b/c the head gasket went. I would upload a video but this site doesnt have anything like that for me to share it. Any ideas what i should look into? Does it seem serious? It's been like that for probably... hmm 4-5 months maybe When the car is off, it will make the clanking sound for about an hour every couple minutes or seconds depending on how hot the car was