I have recently bought a used Randall V2 and the amp makes muffled -midrange thunder like sounds at random intervals (some times you can go through a whole jam session without the noise).The sound gets louder when you switch from play mode to standby and the volume fluctuates. This happens even if nothing is plugged in the amp ,so its not the guitar or cables.Has anyone experienced this before?What are some solutions or what is the problem?