I wrote about this before but I have more info..1990 300 SEL 125000 miles.
I have a squeal that starts at maybe 40 or 35 mph and quits at 58-60. Today it didn't start until 45 and quit at 55. Somedays it doesn't squeal at all. I cleaned out all leaves caught in the cowling thinking it might be that.
Is there any vacuum hoses or something that runs into the dash that may have a leak? I know its not a fan motor and the speedometer that has been eliminated but this problem is driving me crazy. In all my Benz', I have never had this problem. HELP