Strange sound from 355 engine bay

Due to a syncro failure we disassembled the (manual) gearbox of my 355 during the major service and changed the defective syncros, most of the bearings, some pinions and shafts. All the adjustments according to the WSM had been done.
Since re-installation a strange whine noise which increases with engine rpm is present. The sound does not depend on car speed or selected gear, only on the engine rpm and is only present with load (acceleration and decceleration). Quiet without any load and also if the engine is reved up without gear selected.
Removed the gearbox a second time, did all the measurements again, checked the wheels with a microscope for wearout signs- nothing. Everything seems to be fine.
According to my gearbox specialist the sound is not like a typical pinion allignment sound. It must be generatored from something which is rotating with engine rpm.
The sound can be best described as the sound you can hear in the radio if your alternator generates radio frequency disturbance - but at my car the radio is off ;-)
Any ideas what the reason for this problem could be ? Does a defective front / rear clutch bearing generate such kind of noise ?