buzz / rattle is often caused by a broken spotweld inside the muffler, causing a baffle to move with the exhaust pulses.
sometimes the baffle just hits the housing, sometimes it hits other baffles. this type of noise id RPM dependant
clicking on cool down - usualy caused by any 2 hot pieces of metal that are touching. most often heard at a manifold connection, or around the converter. this type of noise most often is nothing to worry about. its simply caused by a difference in expansion / contraction rates of 2 items that are bolted together