Thanks for your comment, Phil.
I see it a bit differently from you, although it doesn’t mean I am right! This is how it seems to me:
Pretty much every bike I’ve owned has a clunk into 1st gear from standstill. I think it’s because there will always be some viscous drag from the oil between the plates. The issue for me is how severe it is on the Mandello - without the brake applied it does a jump forward as well as making a worryingly loud bang.
Obviously it isn’t possible to know whether mine is unusually bad without riding another one.
You say that on none of your bikes is it a clutch issue, always the gearbox. Of course it is true that the clunk comes from the gearbox, but it does so because the clutch is dragging. The dragging clutch means that the input shaft is still spinning, so as the gear on the input shaft slides sideways to engage with the adjacent gear the dogs are moving relative to each other, hence the bang and lurch when they engage and bring the spinning input shaft to a halt.
The sudden stoppage of the input shaft forces the clutch plates to slide past each other. The more reluctant they are to slide, the more torque is transmitted to the output shaft and thus the back wheel, making the bike jump forward.
I don’t think any motorcycle makes a clunk from the gearbox alone. You can prove it by engaging 1st gear without the engine running (you might need to rock the bike). It will snick into gear with the gentlest of clicks.
As well as the noisy engagement of the gears, I’ve noticed that “block” changing down into 3rd and 2nd when coming to a halt can be extremely stiff. Again, this seems to be from clutch drag because the only cure is to blip the throttle, thus equalising the torque on the gears.
I summary, all motorcycle gearboxes are noisy because they don’t use syncromesh. The more the clutch drags, the more noisy the engagement of the gear dogs.
That is how I see it, Phil, but you are very welcome to disagree or prove me wrong! This is just me trying to get used to my first ever Moto Guzzi.