[universeman]

Steelman makes a thing called a Chassis Ear which is a set of microphones on long wires that run to a sort of mixing board. You attach the microphones (magnet or cable tie) to areas where you think the sound is coming from. Then you hop in and put on the headphones, do the thing that makes the noise (really good for road noises) and turn the selector to the various microphones, to see which one is the source of the noise.

Might help you narrow it down.