Make sure the fan is on a flat, stable surface and that nothing is obstructing the oscillation path. Even a partially blocked swing can cause the mechanism to catch.
Another common cause is a loose pole connection. Unplug the fan, hand-tighten all pole joints (clockwise), and restart.
Note: If you removed the front grille for cleaning and then reassembled it, make sure the alignment marks on the grille and fan head are matched before tightening the Allen key screw. A misaligned grille causes vibration.