I can start of by apologizing that my english aint to good so i might have some issues describing my problem.

I where the one that swapped from a 300d (om617) to om602 and 5speed gearbox and the noise where there with the old engine aswell.

Im having a issue with my old w123, basically when im driving between 50-75kmh (30-50mph) im getting a LOUD humming noise that transmits through the whole body of the car (almost like a exhaust laying against the floor). It's almost impossible to tell what end of the car the noise comes from.
The noise is there in 3-4-5gear when the engine produce as much torque as possible, if i put the clutch down in those speeds the noise goes away.
The only way to get thru that speed without noise is by going full out in second gear until 40-45mph and chaging to third so the engine wont produce any torque.
I have driven 90mph with it and the noise never comes back when i come over 50mph.

I changed the engine mounts, transmission mount, subframe bushings, both flex discs (Lemförder), center support bearing and mount, u-joint, driveshafts and propshaft bushings.
Differential mount is ok and wheel bearings is ok.
I tried a different propshaft aswell but i had the same issue with that one.

I really cant figure out what's causing this issue and neither can any other people i speak with so now i turn my question towards this forum.

Regards from sweden