A trick I tell my students to use is "Take a dog for a ride". What I mean is to have someone ride in the vehicle with their head out the window just like a dog likes to ride. Make the noise happen by driving over bumps to make the sound or go around corners if that is when the noise occurs.. Do whatever it takes. The 'dogs' job is to move around and locate the noise while someone else operates the car carefully and makes it do the noise.
With your problem you could have someone ride in the bed to listen for the noise.
This technique will get you in the ballpark. It works great for noisy brakes, squeaky bushings, loose hubcaps, etc.
Rick
