Tube phono question

My tube phono stage has been making an audible noise (sounds almost like a picture tube/high freq. ringing) that can be heard when switching albums. It actually comes and goes believe it or not, Does anyone know what this may be? I've got it grounded properly and the phono preamp is about six months old. Doesnt really bother me it's just that I dont want to damage it and have to send it out for repair. It wouldnt happen to be dirty AC would it? VTL did set it for compliance with my cartridge

Thanks, Chris