Sounds to me like your pickups are microphonic. Some pickups are 'potted' in wax to stop this happening. You can test this by tapping around your pickups with a non-metallic object - a pencil or plastic pen. If you can hear the taps coming though the amplifier, that is the most likely problem. A magnetic object will obviously produce a sound, that's why I'm suggesting wood or plastic.

Some pickups are deliberately left microphonic, especially on semi-acoustic guitars, as it adds to the woodyness of the tone but it means that the guitar can't be played at high volume otherwise there will be feedback. I once had an Epiphone Casino that was like this (P90s) and I couldn't use it on stage at all.

The only 'fix' for microphonic pickups is to have them wax-impregnated but as this is an 'expert' operation, it might be cheaper to replace them with another brand.

Note that as the Spider Jam reproduces a wider frequency range than regular guitar amps, it is possible that you might get feedback in the higher frequencies.