Noise in gearbox

Just floating this for ideas on a mate's scooter. Its a series 3 LI 150 with Casa 185 top end and new crank, bearings and seals. Rides OK and being run in but on idling it sounds like a bag of bolts jumping up and down inside, almost like the gear cogs are shaking and knocking against each other, but its correctly shimmed etc...........
When listening against the hub the sound is exaggerated and sounds worse. we are gonna rip the endplate and gearbox out and take a look but suggestions woul dbe useful ....thanks
When listening against the hub the sound is exaggerated and sounds worse. we are gonna rip the endplate and gearbox out and take a look but suggestions woul dbe useful ....thanks