I agree, all possibilities mentioned here are worth investigating. Curiously enough, I did a 107 km group ride yesterday and I only heard it a couple of times at the end of the ride. (Although, I was worn out from MTB the day before and intervals the day before that--so I was listening more to my pain than my bike.)

I think that I'll start with the fenders as I mentioned--but my time and the weather may limit my ability to test it in the next few days. In any case, I'll let you all know what it was when I figure it out.

But, for now, thanks for all the good suggestions.

Cheers,
Jamie