In a 1995 VW Golf why is there buzzing in the trunk on the passenger side every time you lock or unlock the door?

The central-locking system is pneumatically powered. The bi-pressure pump for the system is located in the rear of the car and can be heard during operation.

Note: If the central locking system stops working but the pump can still be heard, this indicates a leak in the vacuum circuit. This is usually a result of a vacuum pipe becoming detached at a rubber coupling (check connections at the base of the 'B' pillar, under the carpet).