Sudden Loud Buzz on my Tube Amp

I have a Luxman SQ-N100 integrated amplifier (12 watt EL84 based) that has just begun to intermittently buzz at a high volume.

The tubes all look good -- I added two NOS Raytheon 12au7 tubes a month ago, but doubt that's the cause.

I can't predict when it will buzz, but it's very loud and the volume control has no effect on it. If I turn off the amp it continues for a moment. Switching the inputs makes no difference.

Nothing has been altered in my setup and the amp is fed power through a conditioner.

Possibly an intermittent cathode/filament short in one of the 12AU7s.
I suppose I should change them back to the old ones and see if it happens again.
What about the EL84 tubes? Have you replaced them?
Thanks for your input guys. This sort of mysterious problem is the worst part of hi-fi.

I think Atmasphere nailed it. I replaced both 12AU7 tubes and the problem is gone, gone, gone.

I always figure that a tube is either working or dead, which is ridiculous of course.
I had a similar problem with a tube mono amp. One input/driver tube was bad and it made a constant loud buzzing noise.

This is debugging 101, always roll back your last change when a problem surfaces.