Troubleshooting High Frequency Ringing/Buzzing in Amp

[Mason]

Hey yall, I've got an issue with a Plexi clone that I've built, this is the layout that I followed http://site.triodestore.com/50WPlexiLayout.pdf. The amp is about a year old and I believe it's had this problem the whole time. When the amp is turned up loud enough, usually past noon, I get a really harsh ringing/buzzing coming from the amp. To be clear the sound comes directly from the amp, not just the speaker. It's harder to notice when playing the amp loud without attenuation or a load box, but when playing it more heavily attenuated, or using it with my Suhr Reactive Load and playing through an IR it's much more noticeable. Here's a sound clip of just the amp going through the Suhr. The Suhrs output is turned all the way down so no sound is coming through the speaker, all the noise in the clip is coming directly from the amp. https://soundcloud.com/hnmason/plexi-ringbuzz/s-IAkqkIatobA?si=e93f2c569d9a4ce0a7666093e0646ebe&utm_source=clipboard&utm_medium=text&utm_campaign=social_sharing

The frequency of the buzz follows the frequency of the note, but it's a much higher octave or something. This same buzz also makes its way into the signal and I can hear it coming through the cab, or through headpones when using the loadbox/IR setup. This happens on both normal and bright channels and each tone knob has some effect on the buzz/ring.

So far I haven't tried too much. I've tapped on all the tubes to check for microphonics and they all seem good. I've disconnected the NFB wire to rule that out. I switched out the rectifier and bias supply diodes for fast recovery ones in case it was noise from the diodes but no luck. I haven't used any shielded wires in this build so I could try that next. I don't believe it's any loose hardware or something like that. In the clip the amp was out of its head cabinet so there's no issue with the cabinet. I'm wondering if this is either an issue with the OT, or maybe parasitic oscillation? I can start throwing snubber caps in there to see if that helps, but I'd prefer to use that as a last resort in order to not dampen the highs too much.

Any ideas? All help is greatly appreciated!