Chain tension is correct ? If too loose it can jump the front sprocket....and at a worst case, bunch up and punch a hole in the case in front of the sprocket.

 

Chain link starting to seize on the chain?

 

It does sound like the chain slipping one tooth on the sprocket.
Set your chain slack to 30-40mm.

 

The tachometer bounces at the same time as the noise but I can't tell if the clutch was being pulled in at that time too.

If the clutch was not being pulled in the tac bounce would point to chain slip.

The front sprocket could be worn out enough to let it jump.

 

Change the oil and check the used oil and filter to ensure there isn't a gearbox issue.

 

I would be looking at the whole drive chain. Even if it looks good there is a possibility some lazy **** has replaced the chain and rear socket but left the front one in place (remember to tie the clutch lever back before removing the cover).
Hopefully it's this and not something deep in the engine.

 

Check that chain condition/tension and both sprockets.........certainly sounds like a chain jumping a tooth.