I think I've got some form of issue with the viscous drive for the fan on my '89 560SL. It's running fine with no overheating, but there's a noise on the order of a mild howl or moan, definitely changing with RPM but not always directly. When cold and moving at say 35-50 it will rise and fall in pitch even at steady RPM; when hot, it runs directly with RPM. It's louder sometimes than others, and not always noticeable at an easy cruise (say 45 or so).
I can't think of anything other than the fan that won't tie directly to either motor speed or vehicle speed. If as I suspect this is the fan drive, is it telling me it's about to fail, just making noise, or something in between? Does it need replacement, or is it capable of being refilled as an archive thread describes for a newer model?
TIA-

Craig

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Craig Bethune

'97 SL500, 40th anniversary edition

'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)


SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)