Odeon Rigoletto - tweeter buzz

My first thought is a transformer buzzing,possibly due to delamination..If you have spare tubes,which anyone who owns a tube amp should have,you can try replacing the tubes 1 at a time & see if the buzz goes away,meaning it was a bad tube, but If you go through all the tubes & it's still buzzing,I would bet on a transformer,which will mean contacting Leben..
 Good luck..

I would first consult with Leben or one of their regional dealers or a tech familiar with the product. Sounds like 60Hz hum that’s leaking into the signal chain. A DC blocker will quell transformer buzz if its audibly coming from the transformer itself and it’s toroidal which I’m sure the Leben is not.

I would reach out to these guys in NJ, they’re knowledgeable about everything and if they can’t help you directly, they’ll steer you to someone who can. https://skyfiaudio.com/

OP
 Can you tell if the buzzing sound is coming from inside the amp?
 Power up the amp,turn the volume all the way down & put your ear right next to the amp to see if it's coming from inside...
If you don't hear it coming from inside look to a bad tube or ground loop as someone stated...

the buzz it’s definitely NOT coming from the amp. Clearly only from the tweeter. 

what comes from the amp is a very slight hum, a sort of halo that does not bother much. 

Have you sorted through the tubes yet?
 Maybe buy a cheap ground lifting adapter & see if the buzz goes away..If it does your probably looking at a ground loop hum..