My XC does the same thing on washboard rd. FWIW, I read a few forums on Triumph Rat Net where some folks have isolated and attributed it to "chain slap" on the protector in the low RPM range when the tension is lower. Buggers the hell out of me.
Good write up, I went into BMW with the intention of buying the 800gs but eventually bit on the XC after multiple test rides...not disappointed at in the bike. Just wish there was more a community like BMW has.