I just found out that it does not make this noise when nothing is connected to the rear PTO shaft. I had a 3 pt. seed spreader connected when I heard the noise.
Check to make sure that the PTO shaft is centered and doesn't have a lot of play swinging side to side. I had that problem and found that the PTO shaft coupler where it goes into the tractor would nick the stabilizer. It was especially noticeable when I raised my tiller and made a turn.