When I start the car and bring the revs up a little, the aircon stuff gets a bit squealy for a short time then goes quiet. This is without the aircon turned on so it can't be the internals of the compressor (not moving) neither is the belt loaded very much so I don't expect it is slipping. Which is most likely to fail, the compressor pulley bearing or the tensioner pulley bearing? Tensioner is easy to replace, compressor not so sure. 5 litre so has V belt.
If the belt is old it will squeal. If the V part has a slightly glazed appearance, it will be causing the squeal. Fit a new belt.
Just put new belt on on Saturday. None was fitted when I got the car so previous owner must have "fixed" the problem.
How tight do they have to be?
If the belt was off then there is most likely a problem with the compressor or clutch.
It has to be fairly tight, about 20mm up and down movement on the belt over it's longest part.
Tight now. No more squeals.