I guess a sudden drop in oil pressure our engine noise would be a sign of a pump going bad. But the pumps are pretty simple and it's hard to see how something sudden would happen to them that would suddenly reduce oil pressure. I would think they would wear gradually.
I think the pump is fairly easy to replace on your car. Is there a small sump at the front of the engine? If so, I think it comes off for access to the pump.
But before you do that, why not test the pressure with a good guage to verify the problem?
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe