whats wrong with my heater ? vauxhall, zafira 1.8 2000?

I have a 2000 zafia when i turn my heater control from cold to hot i get a clicking noise comming from around the clutch peddle area it only stops when i turn it back to fully cold it sounds like some sort of pump or motor ? if anybody knows whats causing this i would be greatful, thanks, fustrated driver.