Preamp with intermitting Bzzz where to start?

Hi from Europe. I am trying to catch the reason for this intermitting noise that comes out this Euridice Lundhal based preamp (not build by myself)
The noise sound is not heard all the time, in fact there are days where the buzz is not there at all and the preamp is dead silent. On the contrary when it likes to make me jokes, it is buzzing for 1-2 seconds and then goes away for 1-2 minutes and then again.
Shall I investigate only at the tubes or a bzz like this can comes out of other components or for istance from not well done soldering ? I noted many cold joints and fear that this buzzing is the start of a nightmare.
Can it be dangerous for my tube monoblocks ?
I would be very grateful for your best advise from distance.

Thanks

PS.: Tubes where recently changed so I smell no trouble there, possibly.