Another "kinda-newbie" head scratcher

I had your exact issue with a sensitive tube preamp. Traced it to the powerline ethernet adapter. I tried moving the adapter to a more distant outlet, using a long (~25 foot) ethernet cable, but still had the problem. I concluded that it was the modulation of the powerline. It might work with the powerline adapter on the other phase, but it's rare to find both phases in the same room. 

If you have coax TV cable, you might look into using the ScreenBeam MoCA Adapter which sends ethernet over the RG6 or RG59 TV cable. I found this did not have noise spill over to the preamp. Maybe it will work in your situation.

Well that’s what I suspected. Glad you figured it out. 

Hello Eric!  If you have a Grado cartridge with a replaceable stylus (needle) the mount for the stylus actully completes a magnetic path and has a bit of magnetic cream on the edges of the mount that is easily scraped off in handeling the replacement. If that material is not intact, it can cause the problem you are experiencing. I speak from experience, but it was a very long time ago. Good Luck!

Does the volume control affect the loudness of the noise. If so, you may have defective components in the phono stage of the preamp.

@lewm 

RFI is also electromagnetic radiation, usually travels longer distances at high frequencies but is otherwise similar to EMI

Not really.  RFI is just a subset of frequencies within general EMI.  My favourite AI says:

EMI (Electromagnetic Interference) is the broad, general term for any electrical noise that disrupts a circuit, covering frequencies from 0 Hz up to several GHz (including power lines, motors, etc.). RFI (Radio Frequency Interference) is a specific subset of EMI, referring specifically to interference within the radio frequency spectrum, typically between 10 kHz and 300 GHz