Operating the amp without those metal tube shields on the 12ax7's can produce excess noise and even oscillation. Especially in a room with florescent lighting. Some light dimmers can even put noise spikes into it.
Check your input cord. It might have a faulty ground.
Does this happen on one or both input channels? Try using the channel input instead of the both-in. Sometimes crud can build up in the both in input over time causing a faulty ground connection.
