Noisy AC Compressor maybe?

[McDoof]

So slightly interesting one here.
My Passat TDI has a CR Diesel engine, so it should sound a lot less like a traditional clacky diesel. But when I have my AC turned on (always) it sounds like a good old fashioned tractor, clacking away. I had thought it was something like the DSG DMF that was starting to give me trouble, but turned off the AC the other day for some reason and found that after about 30 seconds the noise faded away. When turning it back on the noise started to make it's way back.
It's not an instant on/off.
Is there something in the AC compressor that would do this? I know I can spend the time trying to figure this out, but hoping someone here knows why.

Thanks

[Worms]

The compressor won't have a clutch, just a control valve that internally changes the swash plate angle and varies the displacement of the compressor. They don't click on/off as such.

When the ac is "OFF", the control valve sets the compressor displacement to minimum - almost off, though it does move some oil/refrigerant, generally.

The compressor could be noisier with higher load - at idle, the displacement has to be higher for a given amount of required cooling, than at higher speeds.

It could also be the additional load of the compressor making the diesel noise at idle, louder.

I know on our petrol cars, the L/H fuel readout at idel increase quite a bit with the AC on, rather than off.

[Gordo]

As an A/C unit works by heating and cooling, and there is a delay either way with the noise, it would suggest it's some sort of thermal issue.
It could also be a low refrigerant problem - when was the pressure last checked?

[McDoof]

More troubleshooting required. Have not checked the ac pressure. I've had the car for 4 months. I'll take it into a mechanic. Probably a good place to start.

[McDoof]

I think my AC dignosis may have been a red herring.
I think it may have sounded slightly netter because I had to open the windows when the AC was off. (diagnosis fail )