Lukedog:
You have received good advice above...I assume there is nothing loose...
Another test is to lock the pressure switch lever so it doesn't reach the on-off switch. Let it work a few cycles under no pressure and with pressure approaching the turn off point...If the noise persists, check those 2 bushings & pins on the connecting rod. If OK and the noise is still there, then check for wear inside of the big bushings that hold the crankshaft...Check for metal dust or any other metallic debris on the bottom of your SB, any debris is an indication of wear.
My SB had a similar noise and was wearing the belts prematurely about every 15-20 hours...The crankshaft bearings were worn, one side more than another...Changed both (actually all bushings), overhauled everything and the noise ceased and the compressor went back to its as new specs and operation.
These compressors are a working horse and with very little maintenance they go a long way...
Best regards,
AZ