A strange issue with my "new" Sansui QRX-9001. Any ideas?

[MidiMagic]

You can shield instead of moving. Use aluminum foil to make a shield between the transmitter and the element that is acting like a receiver.

Always look not only in the wall but on the other side of the wall.

I had a case like that back in the 1970s. The right speaker of a friend's stereo was humming and no change in connections fixed it. I finally noticed that the speaker was humming when the set was turned off. Someone had hung a new electric clock (ac motor) on the other side of the wall. It was making the wall make the sound.

I had another similar case with a music band. Suddenly the guitar started humming loudly. The guitarist had moved the direct box (for a balanced line to the mixer) next to a plug-in-wart power supply for the guitar pedal. The transformers fields coupled.

 

[allhartfidelity]

And YOU WIN!

The WiFi is probably 15 feet from the receiver, but that was it.

I also tried connecting the receiver to a grounding and it caused even more hum.

Now I have to figure out how the heck am I going to move that router.


Thank you ALL for your help. This group has been a godsend.

Glad you tracked it down before sending it back.
Many others would not have been so diligent.
Good to know about the wifi. My phone does the same thing.