When I turn on the ignition I can hear a wirring noise from a unit that is housed in a foam surround that is situated behind the right hand side cover inside the trunk area. Close to the inside of the petrol refilling unit.
When I switch the ignition off the noise stops.

Any info would be good.
Thanks

the noise is from the pse pump (PNEUMATIC SYSTEMS EQUIPMENT) A/C door locks etc... what happens is usually a leaking vacuum element in the A/C system causes the pump to run like that, the pump provides vacuum assist to the climate control system when engine or supply vacuum is low, if one of the elements is leaking, it will cause the pump to come on to provide assistance. usually it is the defroster flaps that leak, this requires that the dash be removed to replace the leaking elements. another option is to disconnect and plug the line that goes to the climate control system. if neither is done the pump will continue to run and eventually burn it's self out, the pumps can be very expensive. if you fix the leak in time the pump will be spared, if you wait you will likely need to fix the elements and the pump, no sence in replacing the pump alone, the new one will just burn as well.

My car('96 c280)does this too... Can you disconnect/plug the line that goes to the climate control system without removing the dash,etc? If so, where at on the car can this be done?
Thanks

it can be done at the pump, i believe it is the clear line, before you cut anything let me check for sure

Cool, thanks. That's what I was thinking.