Actually, if I remember correctly it does seem like it goes away (for a brief moment anyways). If I unplug it and plug it back in the rumblings happen fairly quickly - if I took a while (switching outlets or whatnot) it may take 15-30 seconds before the rumbling happens. The crazy hum though will happen as soon as I touch the input regardless of how long it was unplugged - and you're correct, I got a refurbished amp.