Bwitched has rebuilt his box to be quiet http://www.talkmorgan.com/ubbthreads.php/topics/267875/Bevel_box_rebuild#Post267875 Another thought on the difference between the BMW and Quaife bevel box is that the bearing arrangement in the Quaife design by virtue of the use of angular contact bearings means the pinion is supported for thrust in both the forward and reverse direction, whilst the BMW box appears to be only supported for thrust in the forward direction.
The pinion thrust on the M3W spiral bevel gear set (l/h pinion, r/h crownwheel) is only outwards from the crownwheel under driving load but reverses with a trailing throttle when car is driving engine, due to helical gear effect (left hand diagrams)
Clearly there is a load point though when the pinion thrust is near zero and possibility of chatter without correct bearing pre-load, as Bwitched found. Referring back to the Guzzi box, note that pinion bearing pre-load is performed (correctly) with solid spacer and varying shims, not a crude crush spacer. Likely this then is cause of what I can best describe as a banshee wail as it happens on very light throttle settings.
Incorrect backlash could also be an explanation. Note this quote from KHK gears:
Is it a problem to use gears with no backlash? Backlash is required so that gears are able to rotate smoothly. Less backlash causes lubrication problem, leading to abrasion. Also, without backlash, excessive noise is produced as gears should contact with both tooth surfaces. Higher precision gears produce fewer problems when backlash is set at a small value, however, no-backlash must be avoided.
Richard
2018 Roadster 3.7 1966 Land Rover S2a 88 2024 Royal Enfield Guerrilla 450 1945 Guzzi Airone