could also be (the common) fan clutch bearing bracket

with engine off - try to rock the pully for the fan back and forth - or with engine on try to (carefully) get a sthescope down nearby bases to hear better

sometimes it's better just to wait for it to fail and be ready

__________________
-Marty

1986 300E 220,000 miles+ transmission impossible
(Now waiting under a bridge in order to become one)

Reading your M103 duty cycle:
http://www.peachparts.com/shopforum/831799-post13.html
http://www.peachparts.com/shopforum/831807-post14.html