If everything checks out with the turbo, you might take a look at the manifolds to make sure they are properly attached. I had a similar screeching sound on my '85 300D about 10 years ago. I just checked the repair order and it states:

"Customer concern: screeching sound at around 2500 RPM under load. Found intake/exhaust manifolds loose on cylinder head and missing hardware. Replaced missing nuts and washers and tightened all fasteners. Sound is gone."

I remember speaking with my mechanic and he told me it wasn't the first time he had seen the issue. The car had 274,332 miles on it at the time (2013).

__________________
Christopher
'06 Mercedes E350 station wagon (silver/black)
'85 Mercedes 300D (black pearl/palomino)
'85 Mercedes 300SD (smoke silver/burgundy)
'79 Cadillac Sedan DeVille

'05 Toyota Camry (because always running is nice)

'85 Mercedes 300D sold back to orig. owner 8-1-06
'84 Volvo 264GL Diesel, owned 2000-2013