I have recently picked up an Orange Rocker 30 and I've been trying some preamp tube swaps lately. I noticed when I was playing last night that during idle the amp makes a rustling noise on occassion. It goes away when I start playing (or perhaps it can't be heard over the playing, I don't know). It doesn't go away even with the guitar volume turned down. It will go away when I switch the amp to standby though. I don't think it was making that noise prior to the tube swap but I'm not 100% sure about that. Could I have possibly swapped in a microphonic preamp tube? What could cause this rustling noise?