Check out your tubes....it sounds like one is working its way out of its socket or you have a microphonic tube...may be time to replace them and have the amp serviced while you are at it. ...Does one tube have a whitish look? inspect them/.....are there any cracks? while it is on...tap them with a lead pencil...something NOT metal...does it make a loud noise? These are all indicators there is something going on with your tubes..Good luck

Sometimes the ¼" plug will cause the pop because the guitar's output jack is loose or the amp's input is loose. Check the nut at the jack - these have a way of loosening and a quick tightening can solve that problem, if it is that problem. Good luck.