Are you saying that there is a noise coming from the axle, when the truck and trailer are parked and there is no input to the brake pedal?

It would be an easy matter to trouble shoot the electric brakes, by simply unpluging the electrical cord from the truck and see if the noise stops. If it does, the brakes are getting an unwanted voltage either from shorted wiring or a bad controller.

At a rest, the brakes are about the only thing in the axles that can move.