Make sure the heads are bolted on tight. Believe it or not they can loosen and it does make quite a racket. Happened to one of our TD guys a few years ago at Carlisle.
Get an open wrench and check the top ones. Pull the valve covers and put a socket on the ones in there. If ANY turn you need a torque wrench. Loosen them all and then tighten them in sequence to 23 pounds.
Get under there and look for exhaust leaks. Make sure there are not at any of the pipe flanges, or in the slip joint. Once you're sure everything is tight and nothing is leaking, THEN buy a new exhaust system.
If you do that, buy new oil line too. You want those running back (toward you) and outside the sheet metal under the engine. Go the long way around to the oil cooler and back.