Screeching noise

I have a 2001 Ranger double cab XLT 4X4. 2.5 litre turbo diesel. Approx 85,000 miles on the clock.It has begun to make a screeching noise which rises and falls in pitch with the engine speed and seems to go when in 4th or 5th gear. I rarely use my A/C. It doesn't get any worse in the cold or heat and can start when I first start the truck or start 5 mins into a journey. I changed the turbo this year, so don't suspect that. Belts look fine, but then I'm no mechanic. Had a mechanic look over it-he says major strip down to find source or wait till it breaks!