Noise from your amp can be from the outlet, the electric transformer and other signals that may interfere with your reverb tank. If you tried turning the tank 180 and it still makes a bit of noise, then you might want to add metallic shielding. From what I understand reverb tanks are kind of like pickups, so they'll pick up some of the interference from other electronics too.
If it's white noise, would you say it sounds more like a hum? Or is it more of a buzz?