koiroy wrote on 12/31/69 at 16:00:07: 
Rocks Greg?
I must have missed this....yes, rocks. As Al states above they can get in there if you just pull into a gravel driveway...and in my case, I live off a gravel road.
The biggest reason is that the rear fender overhangs the swing arm and small rocks can bounce around the fender, hit the swing arm and get into the belt. It doesn't happen very often but it is possible. I hear them going "ping ping" pretty often on my street, but have never found any damage.
Since I removed my belt guards (and all evidence of their previous existence), I fabricated a "mud flap" to the front of the fender that hangs inside the swing arm to keep stuff off the swing arm.