I've got a squeak

I'm hoping you guys can help me out. I've got a 2001 Xterra SE 4WD. When I am the only one in the vehicle and I'm not carrying anything there are not problems. However when there are two or more people, or I'm carrying some weight in the back every bump I go over causes a squeak. It's hard to locolise it but it seems to be coming from the engine compartment.
I've had my truck in to the dealer about three times now with them looking for this, but they can't find anything. Does anyone have an idea?

Thanks

Sounds like your leafs. Some guys put various types of lub back there to quiet it down. I just turn up the radio .