Gearbox/clutch noise

I have an intermittent noise coming from the gearbox area of my old Mercedes with a Getrag 5 speed box.
It's a rotational scraping bearing type noise. It can start when driving but it's not related to how long it's driven. It can be there when car is first started or develop anytime after car is warmed up.

The noise is present whether clutch is engaged or not, but it changes pitch when pedal is pressed or released. The gearbox operates as normal with no other issues apart from very slight judder when releasing clutch pedal when cold, for first minute of driving.

I was thinking either release bearing or input shaft bearing but wouldn't the noise go depending on clutch position if it was either of those? The fact that it's happening infrequently is also a bit strange.

I was going to change clutch first before delving deeper into gearbox. I have a spare box if things deteriorate.

Any ideas of other checks I could do before taking things apart?