The buzzer is the low pressure warning buzzer. If the air pressure in the reservoirs is too low (i.e less than about 70 psi) the buzzer sounds and the truck will not move, as there is insufficient air pressure to release the brakes.
You may not have heard this buzzer before because you possibly now have a slow air leak somewhere in the system which is causing the pressure in system to drop when parked up.
JD
May 2008