Does the clicking go away forever (until you turn the amp on again) after you push on the tube retainer? You might try switching two of the tubes around and see if the problem persists. Typically, "clicking" is indicative of an intermittent short in a pasive component like a resistor or capacitor associated with that tube. But 300T's are all still fairly new, so serious component degradation shouldn't really be a factor. Aside from checking all the obvious stuff (bad cord, high RF field, etc), who knows? Might be worth a trip to the tech if the amp does it often enough.
