Tighten down the two bolts holding the assist arm to the gate. If it is like when mine was doing it, the bolts were just loose enough to cause it to pop and annoy the hell out of me.
Bingo! I had that damn clunk after only 6,000mi. Thought it was normal until I was pulling apart the sub enclosure to wire my backup camera and noticed the bolts were about to fall out! Tightened em' up and quiet like the day I brought it home!
Tighten down the two bolts holding the assist arm to the gate. If it is like when mine was doing it, the bolts were just loose enough to cause it to pop and annoy the hell out of me.