What part is this? a6 tdi 3.0 quattro

[Audia6tdiquattro]

Hi

Heard a pulsating sound coming from engine bay when ignition is on and also when engine is on - pulsating continues when engine off - took off cover and the pulsating is coming from the two grey cylindrical things in my picture - any idea what they are please?

 

[zit501]

I believe it's the coolant valves for the HVAC system. They do go faulty and aren't too difficult to replace. There's also a video on youtube of a guy cleaning the valve plugs on one to get it working again, which can be done without removing the entire valve body: