Hello mjfulk and welcome to our forum...........
Does your aftermarket header have (or retained) any heat shields .. ?? If so, that would be the first thing I would check (to see if they are lose or hitting the frame).
The next thing I would check, is to see if any of the under panels are lose.
The next thing to check is to see if any of the exhaust pieces oven come close to something else, especially when the engine is torqueing over.
humfrz