Main Knock?

[mach0415]

I have been driving my newly acquired 300sd for a few weeks now. I do not remember hearing this before, but between 1200 and 2000 rpms (sweet spot is at 1500 rpms) I hear what sounds to me like a main bearing knock. It is of lower pitch, happens on accel and decel, but seems to be ok at idle and above 2000. You can only hear it inside the car. I locked the throttle in at 1500 rpms, hoisted it up on a lift and listened with a stethoscope. I did not hear it anywhere with a stethoscope. Oil pressure at idle is 2 1/2 bar. I recently did a service on it and found oil to be metal free. Is it possible to have exhaust resonance cause this? I inspected the exhaust, but did not notice it grounding out on the body anywhere, however, it really vibrates at the flex pipe from the turbo at this rpm. Is there anyone else with an SD that has experienced the same concern. Also it nails at idle. Is it possible for it to nail at 1200-2000 rpms?

Service included
- valve adjustment
- lubri-moly diesel purge
- fuel filters replaced
- lube, oil, filter