What exactly does the EGR valve do, and why do i hear a clicking noice from my EGR solinoid, all the time?

 

The EGR admits a small bit of exhaust gas back into the intake air/fuel under certain operating conditions to cool the combustion temperature to reduce the formation of nitrogen oxide compounds (air pollutants).

I've never heard the EGR solenoid clicking, but the mixture control solenoid clicks constantly.

 

If there's a temperature sensor in the base of the EGR, it will click when hot exhaust gasses pass it.