Screeching noise in low gears

[rompom]

I recently bought a V-reg ambulance for a campervan conversion. It's a Transit 190 diesel cab (2.4lt D engine) and a fibreglass back. It has 166k miles on the clock and MOTs to back that up, but it hasn't been serviced in 5 years (35k miles).

When I'm in gears 1,2 or 3 there is a nasty screeching noise when the revs hit 2k-2.5k, which gets significantly worse when I take my foot off the accelerator. It sounds to be coming from directly behind the glovebox. This is in conjunction with a metallic clicking noise which speeds up as the revs increase (I'm assuming the latter means new tappets, but thought I'd mention it too in case someone on here knows better!)

Has anyone had experience of this/know what the problem may be? If so, what should I expect to spend getting it corrected?

Many thanks

[Vee4]

Are the drive belts OK?