Another Air Conditioning Problem

So the AC on my 2005 Pluriel TU5JP4, is not working. It was low on my list of things to do for my restoration. Now I have started to debug the problem. The climate controls seem to work ok, and I can hear the clutch activate when I turn the AC on, but then I start to get a high pitched scream and turn it off - scream stops. When the engine is off, I can turn the clutch face manually with my hand, which i gather means that the compressor is not seized. The auxiliary belt is properly tensioner and does not seem to slip. Any ideas as to the cause, or where to look next? I was thinking that maybe it was a gas problem, and found a Utube video on checking the gas at the valve, I have not done this yet. Something else? Thanks

If the "squealing" is coming from behind the dash, then the system is low on refrigerant. If the clutch still engages, then there is still a bit of pressure in the system.
The squealing will be from the TX (thermal expansion valve) in the evaporator. It may start of squealing and then go quiet. It wont cause any harm.

If the squealing is coming from the engine bay, it could be the electro clutch in the compressor slipping or the idler/tensioner bearing.

Typically with refrigerant leaks you get associated leaks of oil. So finding leaks should be easy (just look for oily patches). HOWEVER I have had 2 C3's with refrigerant leaks and not one drop of oil leaked out!!!!!!!! While you still have refrigerant in the system (pressure) go over all the A/C hoses and connections with a mixture of dishwashing liquid and water. Use an old window cleaner or other "spray" bottle. Look for bubbles.

Thanks. It is coming from the engine bay. I don't think it's the auxiliary belt tensioner, as I just replaced and retensioned that belt about two months ago, but will check it. I do get a brief chirp when I start the engine, so it is feasible.