Noise in left channel of Audio Innovations ai500

I started hearing some noise in the left channel of my Audio Innovations ai500 about a week ago. It sounds kind of airy - kind of like if you were to say "shhh" very quietly, but enough to be audible. The noise doesn't increase with volume.

I moved the power tubes (EL34s) around - their sequence, that is, just for the heck of it - to see if that would do the trick, but (and not that I really expected it to) didn't do a thing.

I've made sure all equipment is grounded.

There doesn't need to be any source attached to hear the noise.

I've swapped speaker wire and interconnects - reseated all tubes (power, as I state above is via EL34s, the others are ECC83 and 88s - of the five (in a half moon arrangement in front of the EL34s) the two left are phase inverters, the center one is a line driver, and the two at right are the phono pre).

The noise is one that is heard through both channels when the tubes are just heating up - it vanishes to complete silence on the right, but the left sticks around.

Any ideas?

Thanks,
Mark