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.
98 Riviera SC3800 All stock except gutted air box.
1970 Buick GS455 Stage1, TSP built 470BBB, 602HP/589TQ
Best MPH, 116.06 MPH, Best ET, 11.54