Help: Unidentified high-pitch sound

Thanks everyone for the suggestions. The Mod was my first venture into tubes so I'm new when it comes to any issues with them. I'll definitely check the tubes first.

I was turning off my system and noticed it disappeared when I turned off the Bel Canto

If that was the last thing you turned off then its the amp. If its still there even with everything disconnected from the amp, confirmed.

Uh, also the sound will go away when the amp is off as there will be nothing to amplify the signal.
Have you moved cables or wires recently? Make sure that there is no RFI between the cables and you can help yourself by having the cables cross at 90 degree angles, or keep them a decent distance from one another.

I am not sure what brand tubes Dan is using in his preamps but the original tubes in the 9.0 were Philips 5687 tubes. Those tubes were late 1970's or early 80's tubes and frankly, not the most musical. If you are the original owner be happy that the tube lasted a year. The 5687 tube runs hot and like everything else man makes, it breaks. Like the others before me, yes, check the tubes first. Having owned a 9.0 I ditched the stock tubes and went with Raytheon 5687 tubes and never had an issue. I did have an issue with the preamp itself though and mine developed a hum or a whine after it was on for about 90 minutes or so. It had to go back for repairs. It did not do it when I first owned it. It started after about six months and fortunately is was still under the warranty. Since this preamp was in place for awhile and there were no issues previously, I do not think you have a ground loop. I think your amp is a Class D amp and I have had impedance issues with my tube preamps and class D stuff. But then again, you said you have been using this system arrangement for awhile so again, perhaps not an issue.
Go with tubes. And if you buy NOS tubes for the preamp, buy from a reputable dealer. You might pay a little more but you won't be spending money on junk either. I have seen a lot of counterfeit tubes from certain parts of the world and the tubes that people think they are getting are nothing but rebranded POS tubes.

If it does move with the preamp output swap you could then go further.
Open up the preamp( powered down of course) and swap the tubes themselves L and R.
While in there you might possibly find one tube was loose in its socket.
Anything is possible but I would definitely start at the Modwright if it was myself.
Good luck.

My first point of suspicion would be the Modwright preamp with a tube going bad.
I would start there, swap the main pre output L & R to the power amp and see if it moves with it.
If so it is possible a new pair of tubes are the order of the day.

Good luck and stay safe

Hi bluorion, It could be anything but the speaker. So now you have to play detective. Start at your sources buy switching right and left channels.. one at a time. The unit that swaps sides is the problem. If the sound does not change sides then switch channels on the preamps inputs one source at a time testing all the ICs. Then on the preamp's outputs. If the sound changes sides you have a preamp problem. If not then switch sides on the amp's inputs. If the sound switches side you have an IC problem, if not now swap channels at the amps output. If the sound changes sides you have an amp problem. If none of this works (which it will) you have a big headache. I suspect it is either the preamp or the amp.

Good luck,
Mike