hi, played my bass for a couple of hours and it is working fine. I unplug everything as normal however when i set it up again, 30mins later, i get a horrible humming/buzzing sound right from the moment i put the jack in till i take it out even if no strings are touched. Cannot play any of the strings either as all i get is this hum. i changed the lead twice and also had a friend test the amp with his guitar and it worked fine so it must be a problem with my bass. Any suggestions on what to do?