I think maybe my thread was the one you're thinking of. I took mine apart and replaced the front bearing. The brushes actually looked pretty good. But the front bearing was toast. Yours may be going away on you, and rubbing internally somewhere on the armature. Here's the thread:
http://www.peachparts.com/shopforum/showthread.php?threadid=61632

__________________
past MB rides:
'68 220D
'68 220D(another one)
'67 230
'84 SD
Current rides:
'06 Lexus RX330
'93 Ford F-250
'96 Corvette
'99 Polaris 700 RMK sled
2011 Polaris Assault
'86 Yamaha TT350(good 'ol thumper)