The wife's S 10 blazer hitch used to do it to. It was the slack around the receiver on it. ( grab the ball and shake it , and you'll hear it. Did I just write that. Lol)

It only did it when the tongue weight was low. Then it happened on every bump in the road. Thing would drive you nutts. I was going to weld a couple of beads around the receiver and grind them down till it was tight but never got around to it.

Never could remember it till it was hooked up and bumping down the road. My son later rolled it.

The new truck doesn't do it thank god. It must be certain brands of hitches.

Another thought would be to cut pieces of milk jug or other thin plastic and slide into it as a shim.

Thanks for all the help fellas. I pulled it outa storage today. Guess what? No clunk at all.

So I repacked my bearings, put the batteries back in, and cleaned her up. Man did it ever feel good. I thnk I'll do it all over tomorrow.  I think not. I'm going fishing tomorrow. Yea!

I pulled my boat down to fuel up today so I could take it out of mothballs. This was the. First time I've pulled it with the new truck........and I hear the clunk.

It's my hitch moving around the receiver......I'll see if I can find something that fits tighter......or I will shim it up by making a shim out some hard rubber.

Chris, I was thinking that was my problem as well.