Disco 300tdi squeal under acceleration

Hi chaps and chapesses.
300tdi manual.
Had a water pump fail recently which I changed and replaced the belt, 7v.

Soon afterwards a squeal developed which I assumed was the tensioner bearing. I've just replaced the tensioner and I've got the same noise.

The noise sounds like the squeal that occurs with a dry bearing and it happens when I'm accelerating. It disappears as soon as I lift my foot off the throttle and reappears as soon as I start putting my foot down again. It makes this noise in all gears but it doesn't start as soon as I release the clutch but once I'm moving so I don't think it's the clutch release bearing.

A few weeks ago I topped up the gearbox. I was told by the carparts shop that 300tdi's that it used ATF in the gearbox. Is this correct?

Edit:- I've just tried the penny trick, cleaning the pulleys and belt, reversing the belt and nothing has worked.
It starts at low speed now as a squeak-squeak-squeak and get faster as I accelerate. It doesn't happen in reverse so I assume it must be a lay-bearing in the gearbox.
Unless anyone else has any other ideas. Thanks for looking.
P.

Any suggestions gratefully received.

Had the same problem with mine ,I changed lots of things & still had the chirping noise. I bought a tin of belt dressing and sprayed that on , not had a problem since

Thanks Thomo, I'll pick some up tomorrow and give it a try.
Nothing to lose but a chirp

layshaft bearings are diagnosed by it been quieter in 4th not reverse as 4ths the only gear layshaft has no torque going through it, look at ujs too

Had chance to check it properly today.
Definitely the rear UJ on the front propshaft.
Thanks for suggestions, especially putting my mind at rest regards gearbox.

Was going to say its a UJ just had the same on mine