Gearbox noise, loud screeching?

My td5 99 (166000m) is making a loud screeching/high pitched grinding noise when changing gear,
This doesnt happen all the time and used to do it from cold only, it has now started to do this after a run every now and then.
This happens when the clutch peddle is pressed and it is really quite loud.
Hopefully somebody can advise me what it could be?
Thanks

Clutch release bearing

I agree with Mad Hat Man

Not a particularly expensive part and a simple job, just a faff to split engine/gearbox

If yu are gonna do it - get the stronger release arm at the same time .