Yes, turning off the reverb stops the noise/roar.
The sound is not the normal hiss you hear from the power being on. It's hard to describe. It's like the reverb starts acting up and begins to roar. If you shake or hit the amp you hear the springs in the tank rattle and make a loud noise, then the "roar/echo hiss" stops. The reverb goes back to normal, then it will happen again. Almost like some electrical interference brings it on. I really don't know how else to explain it.
I never tried unplugging both connections from the tank, because the unit is all put together when it happened.
And, no the sound is not like an AM radio in between stations. The AM sound is there as soon as you turn on the power. When this noise is happening you can't hear the reverb effect while playing guitar. It's like a thunder and lightning storm. Give the amp a tap so you shake the reverb springs and the noise goes away and you can hear the reverb effect with the guitar again.