I'm getting a rumbling noise, similar to the sound of a distant helicopter. Its only happening in raw and vintage modes in both the red and orange channels, as well as both modes in green channel. I only have 2 of 3 rectifier tubes installed (1 shattered in shipping when I received amp). I noticed my second 5U4G tube went out and wondered if that has anything to do with it. I run the amp in diode mode at all times. I did the tube tap test and all of my pre and power tubes seem ok. Any ideas guys?