2000 Saturn LW2 Noise Problem

Please help me with my 2000 Saturn LW2. I purchased the vehicle last year with 119,000 miles on it. I had to replace the electronic ignition switch last August and the water pump in December. Since the timing and serpentine belts had to come off to replace the water pump, I had those replaced as well. Then, the tensioner pulley for the timing belt, which was not replaced, went bad within two weeks of getting the car back, and I had to replace that. About two weeks ago, the car started making a rattling/tapping/almost like a diesel-like sound. My mechanic says it's coming from the top of the engine. It's a 24 valve V6 DOHC engine, if that helps any. He changed the oil, the plugs, checked the timing and serpentine belts, ran an emissions diagnostics to make sure that the catalytic converter wasn't a problem, checked the oil and fuel pressures and all everything's good. He said that the car passed the emissions test. Accompanying this diesel-like tapping sound is a loss of power. My mechanic has no idea, and we are at a total loss. Have you ever heard of this type of thing before or have any ideas? One of his employees is going to listen to it with a stethoscope, but they don't think that this is going to really help. Thanks so much for your time. NOTE: the car had a clean CarFax when purchased, and, per my mechanic, the previous owner obviously maintained the car very well.