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.

Thoughts?