Never heard an amp do this before, ideas?

Alright, the faraday blanket lends support to the "external radiation"/Wi-Fi theory. 

As a (possibly amusing/enlightening) side story, I had tried substituting other equipment as part of my logical deduction, detective methodology and found that a solid-state version didn't suffer the same noise (both pre and phono) and so, the units were obviously the problem (I thought/but really, just better shielded). My technician was not able to duplicate my problem but charged me anyway.  When I finally figured out what the problem was, I shared that with my technician, hoping and even suggesting that maybe he shouldn't have charged me. His wise response: "Turner, it's always what you don't know that costs you."

Think "out of the box/system" if all else fails. 

There's an Android app NetSpot that will show the level of WiFi RF in the area.

Sitting right next to home router is usually about -40dBm.

There are relatively inexpensive RF detectors available. The Best RF Detectors for Detecting EMF and RF Signals in 2022 – SPY

BUT, even if you find the source, getting something done about it may be difficult.

Since the sound is present on all inputs, you could pull the power tubes [labelling them first to restore to the same socket] to see if that makes a difference. Only power the amp for a couple of minutes as B+ will be high without the power tubes.

@hobbes101

This may seem silly, but it can actually work quite well.

To test for RFI you can use an old transistor radio. Just turn it on and adjust the dial so it is in between stations. Turn up the volume and then use it like a geiger counter. When RFI is detected the radio will emit static.

Tony, I'm not sure the Smithsonian will lend one out. Good idea though.

Have you tried a Ferrite Ring Core RFI EMI Noise Suppressor Cable Clip, you can find them on Amazon and eBay? I had a serious issue with rfi in my system, I could hear the local am radio station broadcasts. I bought a package which included several different sizes that I was able to put on all cables attached to my amp and throughout my system. It offered me quite a bit of relief. I hope you figure out the problem, I know what it frustrating experience this can be.