Hi James,
I just have to wonder if it isn't the Good Fan Belt Elves singing "Jingle Bells" to you? If the sound goes away after Christmas, that's probably what it is.
Other less pleasant possibilities are water pump bearings failing, alternator bearings failing, fan belt(s) failing, mouse in the works. I seem to think your car doesn't have a fan clutch, so that's out. Loose bolts holding the fan on might be a possibility. This would all be the work of the Evil Fan Belt Elves.
With engine off, try to wiggle the fan front to rear. If there is more than slight movement, suspect the water pump or fan bolts. Other than that, it may just take a bit of poking around to figure it out.
Merry Christmas, and I hope it's just the Good FBEs!
George Davis
'69 280 SL Euro manual