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.


PS.: Tubes where recently changed so I smell no trouble there, possibly.
Perhaps a loose connection in one of the interconnects, or the input/output jacks. If the cables have been twisted during removal or installation, you may have loosened the nut on the back of the jack. You should also check any controls (knobs, pushbuttons, etc.) for loose connections.
Get Someone you don't particulary care for to move wires or components around while the unit is running so as to find a bad solder joint.
Why not start by replacing the tubes? How old are they?