Just tighten your chain! LOL
You may have a piece of debris behind the wheel coupling.
Remove wheel then remove the coupling retaining ring, coupling and damper.
Check the wheel bearings, might be time to replace.
FIRST!! Check your final drive oil level, Have you changed it?
Do you hear it more on the left or right?
Thanx for your reply Lance!
Yes, I changed the final drive oil. The sound is definitely coming from the right (brake hub) side. When I had the wheel off I checked the bearings by turning them by hand and they seem to turn smoothly. Is there any other way to check the bearings?
I studied the wheel assembly in the Clymers. I agree, I think the sound is most likely coming from a piece of debris behind the wheel coupling. The sound occurs each time as the debris falls from top to bottom only once per revolution. Very definitely something is tumbling inside behind the wheel coupling.
This sound has been there since I got the bike a few months ago. I've put several hundred miles on it without any problems. I'm tempted to do a test ride to check for problems before I remove the rear wheel again.
What do you think?