Technical Support wrote:
The beep can come from a misfire, which could be caused by a loose tube.
If the tube has loosened and has been used, I would remove the tube and look at the banana plugs on the legs to ensure they have not been charred by arcing. This will have a burnt, charred look. You may have some dark discoloration of the legs, but that is OK if it is not ashy. If the tube has arced and has damage, the head will also be damaged. This will require the unit to come in for repair. While the tube or a new tube may continue to work in the light with this damage, the damage will worsen and damage any new tubes installed.
Also, there will be some dark discoloration inside the glass. This is normal and is not indicative of a problem.
Just to be clear, the beep continued as long as the power switch was on till turned off. It then started as soon as the switch was turned on again.
I'll check the plugs tonight.
Thanks
James