I would put the ride on hold
It could be anything from a slightly out of adjustment swing arm, loose drive shaft coupling, worn u-joint even the transmission tail shaft bearings gone south.
How many miles on the bike?
What kind of service history, particularly spine clutch drive shaft service have you done?
Does the bike get loaded up for touring a lot or has it in the past/
You could try loosening the rubber boot from the back of the transmission and pull it back to see if the bolts are loose or there's a sign of what's making the noise.
None the less I wouldn't ride it until you at least know your not making a problem worse.