Assuming the noise slows down as the truck does, and it goes away when you release the brake pedal, I'd start with a brake inspection. A common cause is a warped brake rotor and grooves worn into the mounts the brake pads rest on. When the brakes are applied the pads are forced against those mounts and a warped rotor will make them catch on the grooves and make a clicking or snapping noise from sliding back and forth. Often a front brake job or a simple rotor machining will take care of it.
Your mechanic will also check for loose wheel bearings that allow the rotor to tip a little and rub on the splash shield, and other things like that.
Wednesday, October 3rd, 2012 AT 7:12 PM