Hey guys the GF noticed a thunking noise when I hit the back brake yesterday while riding. Then I heard it as well. Put the bike on the lift and while running with back wheel elevated and only when hitting the back brake quicky and sharply I could repeat the thunk. Only with a quick sharp hit on the brake. For awhile now I've felt slop in the drive drain when letting the clutch out, or an unusually clunk with the occasional starter kickback. Going to check the primary chain adjustment here shortly, but I don't I'll get that lucky. Rotor, axle nut, caliper bolts, sprocket, wheel bearings have all been checked.

Any ideas?