i'm curious about this alarm, would this be the parking brake alarm?
i have seen in several cases where the switch on the parking brake handle will go bad or the switch arm breaks off causing the alarm to sound even when the brake handle is set.

if the alarm is going off even when seated in the operators seat then the PDS system is malfunctioning. Make sure the seat switch is operating properly, check the parking brake switch for damage.
Also when the alarm is sounding check all lights on dash and make sure none are lit up, if they are it should be determined which light is lit and what system it is associated with so you can determine where the fault is.

going on what edward is saying, if your brake linings are worn out or damaged there should be an indicator light on the dash, if any problem is detected the light will flash and an alarm will sound. Is this happening? Does your truck have the brake indicator light and is it flashing while the alarm is sounding?