I had a similar problem after having a muffler shop finish my install by welding the joints. Before I got there I did not experience any "clunking" but afterwards I did. I suppose the heat expanded it some but I wouldn't think that much. Anyway, my left rear muffler was bumping against a stiffener in back of it. So I had the muffler shop cut a section out of the stiffener to solve the problem.
