Could be slop in your cush rubber is transferring a jolt up the chain and into the trans. Funny how you can hear things when the throttle isn't twisted, but can't when it is. Jack up the bike s rear, put it in first and attempt to to spin the rear tire and see if there is any torsional movement in the rear hub segments. I'd put a snug TBS rubber in and be done with it, although I have read some creative home fixes posted from members on here. Also, some have reported heard noises, and found the cause to be a worn out chain and sprocket set, check that too. Eliminate what it ain't, to figure out what it is.
