If it's repeatable have someone drive the truck over the stretch of road where the noise is heard while you ride in the back listening for the location. Sound can travel and what/where you think you hear may not be the case. Back in the '60s I worked at a dealer who employed a fellow full time to work only squeeks, rattles, wind noise and water leaks. It was amazing how he had the knack for tracking these things down. I remember seeing him hanging out of a car door with his head almost hitting the street trying to locate a sound underneath. Often a customer would complain of a noise "in the dash" that was actually originating somewhere else in the car.

Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.