Heater problem
2004 Vauxhall Vectra 4 cyl Front Wheel Drive Manual 68000 miles

My air con is working, fans are working on all speeds, but when I turned the centre rotary switch to any temperature setting it makes a clicking sound and then the noise stops, but the fans work constantly during all this. When I turned the centre rotary switch to another setting it clicks about 4 times and the noise stops. The sound is coming from behind the dash..And bonus there is no hot air... I do not have a digitial climate display..