[morzh]

If it starts immediately it is likely related to the section including the power transformer and the wiring towards the 80's filament. There is no other load there. And the current is fairly sizable so I would look at the 80's panel as well - arcing etc.
It is also possible the transformer is simply loose and what you are hearing is the metal plates vibrating and hitting each other in the power transformer's core when under load of 2A by 80's filament. Although then why other tubes do not do this? Their currents are smaller but together they amount to quite a bit, especially the two 42 tubes 0.7A each.