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?

Its not clear (to me) if you reversed your small tubes. What you have described is a sound consistent with a small tube's noise floor - some are lower than others, or with a small tube that may be starting to go south.
Heyyyyyyy, wadaya know! I noticed that (uhem) I had an ECC83 where, uh, (uhem) an ECC88 was supposed to be.

I've since re-tubed the whole thing with Pearl cryo'd SED 6L6GCs in the power section and Sovtek 6922s in place of the previous ECC88s and finally 12AX7LPSs in place of the ECC83s...

All is quite well now!

Thanks a ton for your quick observation - it saved me a whole lot of heart ache (and frustration).