Probably nothing to be worried about.
These amps were made for the technician. They use push on connections through out most of the amp. These alone can make for very crackly operation until the amp "warms up" enough to tighten them.
If at any time it doesn't go away... jiggle the control knobs. You might find that the bass control has gone south and this can cause some very weird crackly sounds as well as sudden losses and increases in volume. If jiggling any control helps out, replace the control.
Hope this helps!!
