Thanks for the suggestion, Q. The fan blades are clear of the shroud, and the sound doesn't suggest anything hitting anything.
It's clearly audible over normal engine and road noise, but not truly loud, and has a definite pitch that varies more or less closely with engine speed--but when cold, the pitch will vary even at constant engine speed.
My main questions:
Are there conditions when it's normal for the viscous clutch to cycle somewhat?
Is there anything other than the fan drive that would sound like this? (I can't think of anything.)
Is the fan drive on my car is the same as discussed in this thread:
Clutch Fan on 300D 124