Any suggestions on what is causing a clunk sound whenever the truck and trailer are not on level ground?  Truck is 2000 F350 V10, non-dually.  Trailer is 2000 Jamco, 4 horse with 5' short wall that's been hauled very little.  Hitch is B&W.  Truck does not make the noise when pulling the boat.  I've only hauled the trailer with a friend's truck once and can't remember if I heard the noise.  We drive very slow and careful...65 on highway is max.

Thanks in advance for input!

 I assume the boat trailer hitch is a rear receiver, and the horse trailer is a bed mounted GN ball. I would think that if one were making noise and the other wasn't, the shared truck's suspension components wouldn't be an obvious culprit.

Like Paul said, it may be the GN hitch, particularly if it is retractable. It may be something as simple as loose bed bolts. Get a couple of people and rock the truck by pushing on the top of the bed from side to side. Get some people together and jump into the truck's bed. If they do a coordinated wave action, you may be able to produce some noise and discover what is loose.

With someone riding in the bed, drive down a rough road. Your passenger may be able to specifically hear the point from where the noise is originating.

I have a B&W hitch and often hear a bumping noise from the spring loaded u bolts from the safety chains.

Happy dance, Gard is back!!!! 

Thanks for all the suggestions...hubby is going to round up some guys at work tomorrow and start shaking the truck.  And look for anything loose.  Our first GN and we feel like we're learning a foreign language.