What does it mean when my Tube Amp is making a wishy hiss sound that goes away and comes back randomly?

My Peavey Valve King 212 will randomly make weird wishy sorta untuned TV channel noises, not like white noise or distortion. It will do it without any inputs, and it will come in and out, sometimes not doing it and sometimes it will constantly do it. I imagine this is a tube issue? How do I check it out?