The rocker arm needle bearings are failing, most likely the thin flange of the bearing case has cracked out, allowing the needles to 'escape' through the gap in the blocks or pillars that keep the rocker arm centered on the valves .
I would suggest removing all of the rocker arms to find out which one(s) are the problem .
I had this happen to my '81 R65 in under 10,000 miles .