Twin making a new unwanted noise...

[psalt]

to go more in-depth it's only when there is nothing plugged into the amp so I’m more really just wondering if there is something wrong or if it's just the volume I have the amp at. Master volume is at 4 and the vibrato volume is at around 8. Normal channel is all at zero (It's a later 70s silver face).

EDIT: I should mention the fact that it didn't always do this, it's a recent thing. I also have a SM57 sitting in front of it that's miking the amp to the PA.