Engine Noise

I have an 89 Riviera I just bought. The engine makes a metallic resonant rattling sound. It sounds like a heat shield or some thin metal sound coming from the engine or exhaust system. It resonates with the engine speed , smoothing out some at about 1700rpm. I have had the exhaust checked and it does not appear to be the problem.
It seems to be coming from inside the engine. I pulled the serpentine belt to eliminate any of the accessories and no change. I have read that the motor uses a balance shaft which has a thin metal cover over it. Is it possible the cover has come loose and or will bearing failure create the same sound. The motor has 30,000 miles since a complete rebuild. Starts , runs , idles good, great oil pressure 48psi. Any feedback ?

A healthy 3800 should make 60 psi + oil pressure at low RPM. It could be a rod bearing. Pull a plug wire one at a time. The noise will lessen when you eliminate the right cylinder from firing.

Thanks Larry I will give it a try. Oil pressure is 47psi @ idle . 52psi @ 1700rpm .