I dont know that there is a zerk but I know that when mine did that it was because the pto clutch gap needed adjusted.
I copied this from another forum.
"Take the grill off the front of the tractor, it snaps in place so just pull it off from the front. Use a 0.018" flat feeler gage. You will see three hex nuts on the face of the clutch unit, spaced 120 degrees apart. Along the side of the clutch near each nut, is a slot, (three total) that's where the feeler gage slips in. Slip the feeler gage into one of the slots, and tighten or loosen the adjacent hex nut until you achieve a slight drag on the feeler gage. Repeat the process at the other two locations. Then recheck all three in order until no further adjustment is needed to feel a slight drag on the feeler gauge."
I would check the air gap on the PTO, as it sounds as if it is too tight. I would follow the instructions posted by EJH. Small adjustments, like 1/4 turns max at a time on all three nuts as they are sensitive.