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.