Have you ever had an oil leak near the AIR pump? (head gasket or timing cover leak?) I had that muffled noise as well, until I discovered it was a bad rubber elbow hose that connects to a metal pipe which in turn goes to the head. (oil had expanded and ruined it) There are actually two of them down there and they are elbow shaped.
Check those for leaks before you conclude that your pump has gone bad.
'91 300E, 208k miles