Preamp Feedback RD 112 50

I am getting a feedback on only the distortion channel when I have the master or the gain at a 9 or 10 setting. The weird part is it happens even when there is no guitar cord plugged in. Any thoughts? ps The clean channel does not do it at all and it makes no difference if the reverb is on. I also changed tubes.

It sounds like it could be a microphonic component or connection inside the amp.
OR
It could be oscillation you are hearing, not feedback. This could be caused by a bad connection or component as well.
Does adjusting any of the other controls change the character of the sound you are hearing? If so, what is the audible effect?