1985 300D, approx 350,000 miles. I have a bird noise under load around 2500 - 3000 rpms.

This is not a single pitch whistle, but more like a warbling sound that increase in volume and pitch as rpms increase, then drops off after upshift or I let off the throttle.

I have checked the following:
Air filter hose connections
ARV - put nut under spring to increase pressure and disconnected vacuum
EGR - disconnected vacuum

My next check would be the inner and outer sealing rings between the turbo and the boost pipe. How likely is this?, or do I need to look elsewhere.

Thanks, Grezer