I tracked it down to the faces where the upper and lower bevel bevel shafts join, these had starting to wear. This in effect allowed the whole upper bevel assembly to have some play allowing the cam to 'push back' and create a god awful slapping noise on the cam.
20 years later, I've got a replacement set of bevel gears
The bevel shaft diameter is 14.97mm and the sleeve inner diameter is 15.04mm
Does anyone here know where I could get an undersized sleeve to ensure a tight fit between the faces? Has anyone here had the same issue?
Thanks
Steve in sunny Surrey UK