Well two and a half years on I still get this "low load rumble" and now I know why.
Mates GS1200 has the same feel, but much lighter/softer than Bella. It has uprated Ohlins shocks fitted.
My work ute (SUV) developed the same symptoms 5k kms after a new tail shaft centre bearing was fitted. Got progressively worse next 15k km. This had my mechanic stumped.
Most of the symptoms similar to tyre wear/imbalance problems.
Best mate fitted a 2" lift kit to his 4WD and had the same symptoms. Dumb arsed bush mechanic he is, worked it out and fitted 13mm spacer to his centre bearing mount ................ all good.
Told my mechanic as he was doing another customers tail shaft, it had spacers already on the centre bearing mount. Did my ute and problem solved.
Posted several times in this thread .......... "longer shock/swingarm angle?"
Bella has been raised 60mm rear (ride height) by increasing shock length 20mm, and dropped 30mm at the front via shorter forks and smaller wheel. So input/output angle at the uni has been increased. But the rumble or vibration only seems to happen on light throttle at low revs and usually in the lower gears, That's why it's called "low load rumble".
Solution; Ride it like you stole it! At higher engine and road speeds everything is smooth and sweet.
When I destroy the uni I'll fit a Thompson Coupling.
GRiSO OWNERS - Take Note!
Many here have increased shock length by 10mm to 330mm eye-to-eye, and dropped front forks to the 6th line.
While these adjustments represent less than half those on the Bellagio, you may increase wear (decrease life) of the uni joint/cardan shaft.
Stelvio has longer shock/travel/ride height standard. If this was increased it may also become a problem. Dunno?