Is this a new grill or you had it for awhile. The clunk you heard could be clump of pellets got a bit jammed that the auger just finally bust them up. It happens to me at times when too many pellets fall in the slot.
I am curious if this a new grill or not as if it is a new grill check the wiring to make sure that the auger and the fan connections are in the correct connection if not swap them. Refer to your manual for the wiring diagram.