You can use a stethascope (long screwdriver w the handle pushing your ear closed) to listen to different areas and pinpoint mrtal on metal noises. A piece of 3/8" oil line hose is good for listening to header leaks. If its a coiled hose its even better you can use the curved end to snake around and under the tubes. If its leaking you'll find it w/ the hose. Don't forget to check around the collector junction where the tubes meet for bad welds or cracks.