What is this valves purpose ?

[Djlee]

Hi everyone , can you please tell me the purpose of this valve please , I know it’s a heater control valve but what does it control in the heater ?
reason I’m asking is ,it’s noisy when you turn the fan on and off so just curious to see it’s purpose before I change it

thanks

 

[Old.Tony]

It controls the flaps inside the air ducts to mix warm and cool air to adjust the temperature of the air coming out of the vents. Your noise may be caused by this ("air mix door motor") or by the flaps themselves.

 

[Djlee]

Cheers