If it's a variable-pitch sound as your speed is steady, check a thread on this site under "the howling"; I get a similar noise under cool temp conditions before full warmup on my 560SL, and I'm sure it comes from the viscous fan drive. In my case, the viscous drive appears to go into locked-up mode at too low a temperature; since it cools the engine just fine, I'm putting off replacement till next spring after the storage season.
'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)