It could be a lot of things. A competent repair tech should be able to solve the problem. You can download the schematic from Schematic Heaven or maybe fender's Mr Geargead.
In any case, isolate the problem. Ball up your fist and whack the amp. Does the crackle respond? If so, there is a loose connection somewhere - could be solder or wiring, or could be loose hardware. Wwiggle each tube in its socket. ANy of them make noise when you do? Loose tube socket then.
Now move on to the controls. Do any make static noises when you turn them? They need attention then - clean them, or tighten their nut, or repair their solder. Turn the reverb control to zero. Still noisy? The reverb sort of is off by itself so it is good to eliminate it early or to zero in onit.
Turn all the conrols down to zero. Still noisy? If so, it is in the power amp. Possibly a weak plate load resistor in the phase inverter, or maybe a failing cap. If the noise is gone, then it is from earlier in the amp. ANy control that affects the noise - loudness, tonal quality, whatever - is after the source of that noise.
we can also pull tubes to isolate things. If pulling a tube kils the noise, then the source of the noise was either at that tube or before it. So you need to pull tubes in meaningful order, because any tube you pull will quiet the noise from ANY earlier tube.