When my amp is on i get these strange jittery noises as if there's a loose connection somewhere. Anyone know what it could be?

Reckon you could you describe it a bit more? Is it hissing, buzzing, humming, static or what?

How old is the amp? When did it last get new power tubes? Does it happen on both channels? Have you checked all the connections (speaker, power, reverb, footswitch etc) are okay?