Ok so i noticed a few weeks ago that my 65 DRRI is making a strange sound. Even when everything is completely unhooked, the amp makes a sizzling sound that grows louder for a few seconds then goes away. This cycle happens maybe once every two minutes. It never gets loud enough to notice while I'm playing, and no one really notices it but me when I'm not playing... It's always been road case kept, and fairly new tubes. The only problem i've ever had with this thing is that around a year and a half ago, a "friend of a friend" borrowed it and "Paid me for the rental" by changed my tubes out for me and put 6L6's in it instead of 6V6's. I noticed the difference as SOON as I got it back and changed em back to 6V6's myself immediately... Could this have had some sort of adverse effect on my amp this far down the road??