Maybe dust or dirt is in your amp, but you said it doesnt do it when playing clean. Tube amps easily scream, and they provide alot of distortion. So like Axl said, you can't stand close, or point the headstock to the amp, makes the amp mad. How close are you usually to your amp when playing? And how healthy are the tubes on it?
I had this problem a while ago, everytime i played on distorion, my amp made this "static" sound, or a ticking sound. There wasnt anything wrong with my gear, just other people in the area with their electrionic's. Its really hard to answer these type of questions with alot of advice cause i cant hear the problem...is there anyway you can record using that amp on distortion to capture the sound thats being difficult?