DT50 Topology III ringing noise Extremely annoying!

[Hoghntrpa 3]

Bought a used DT50 2-12 a few months ago. I get an annoying ringing sound (like metal on glass) only in topology III. I checked the tube restraints and they're tight. Only thing I can figure is the tubes' inners vibrating but doesn't make sense to only happen in TOP III. Anyone have any ideas about this? The amp works fine in that TOP but the ringing is driving me crazy.