If the air tanks get to the correct pressure the buzzer will go off. Going back a long way in my memory when the air get up to pressure the unloader valve lifts and keeps the inlet valve in the compressor head off its seat with a jet of air. These unloader valves get a carbon build up inside and unload prematurely if you follow the small pipe from the compressor head you should come to it, it is adjustable. There should be a non return valve in the main pressure pipe to the tank it will be a long connection between two pipes, this also carbons up but can be cleaned. If you take them apart they have springs inside so be carful or you will spend a lot of time looking for them. You can check if it is the unloader valve by disconnecting the small pipe and then see if you have a better air pressure, if the buzzer stops sounding then you are on the right lines. There is a max pressure valve on the tank so that will unload if the pressure gets to high. Clayton Dewandre valves are still available if you shop around.