A guy in Des Moines paid me $40 to look at his truck and it turned out he just left a socket rattling in the frame rail
Spent 20 minutes hunting a phantom clunk on this 2011 F-150 before I found a 10mm sitting right on top of the crossmember where it must have been riding for 6 months, so now I always shake the frame before I break out the stethoscope.