Received a used Axe III… and something was rattling around inside…

[Bruce Sokolovic]

Whatever it is, find out It'll almost surely cause a short sooner or later. Happened to me with a guitar amp.

 

[visualsuplex]

If you haven't found the bolt yet, you may want to check throughout the enclosure to see if it's still in there, wedged in a corner or under something. It could have gotten stuck in a spot and potentially jar loose when the unit is moved later on, which could lead to a short. Maybe give it a few good shakes (when powered off and unplugged to see if it shows up.

I did give it a good shake before firing it up but nothing seemed to be loose.

But you're right - it had to go somewhere and unless someone else serviced the unit, it must still be in there.

fml