I'm looking for some advice on what you all think might be happening with my amp.  I have found when I let a note ring out on my guitar, or any guitar, I hear an odd aftertone that can best be described as the sound you might expect to hear if someone was on a respirator in a hospital - kind of like exhaling or inhaling from a tube, almost gurgling like and hissy.  And it principally happens with the notes right around middle C.  I hear it most pronounced on C but also a half step below and above, though not as prominent.  And I can duplicate the problem on different strings, but always around middle C.

Could this be tube related?  It's certainly not the cabinet, I don't think it's the speaker, and I can hear this at any sound level actually.  I'm not that familiar with tube problems as this is my first tube amp in ages.  Oh, and it happens with any of the topologies or channels, though the extent of it varies depending on the one used.

Any thoughts out there?  Thanks for you input.

Bruce