Wondering what a noise is?

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.

There is a tool you might be able to borrow or rent from an auto parts store that borrows them called the "Chassis Ear". It is a set of six microphones, a switch box, and headphones. You clip the microphones to suspect points, then drive around while listening with the headphones. You can move the microphones around to zero in on the source of the noise. Be aware that many mechanics have never seen or even heard of this tool. Suspension and alignment mechanics use it to find rattles, squeaks, and other noises.