AYC pump?
It's somewhere under the car near the rear diff.
(touch wood) not had a problem with mine so far
You may be able to try an adjustable switch which is a fair bit cheaper than replacing the pump (£2000+ from Mitsubishi) but there's companies now that can refurb them for around £4-500 I think.
Personally I think the adjustable switch is just delaying the inevitable.
edit:- I think this is it