Impossible to say without actually hearing it.
It could be anything from a short in a cord or your guitar plug, volume/tone controls... loose speaker grill or cone, tube problems.. RF frequencies, lightning, ghosts etc etc..
All you can do is wait for it to happen again and work your way from one end to the other to isolate where it's coming from.
Start at the guitar and see if wiggling anything makes it happen.. the pickups, the controls, the plug, the cable... move down the cable to the next piece of equipment and just keep going, checking for shorts anywhere.
If you hear it again, try to narrow down where it's happening in the chain.