Never heard an amp do this before, ideas?

I too was stumped when there was noise in my tube preamp and in my tube phono. Turns out to be the Wi-Fi modem and (especially) repeater boxes were too close to my equipment. I could hear the noise abate as I moved them 12 -15 ft away. It falls off rapidly in an electromagnetic inverse squared fashion ....but if even turning off your internet equipment doesn't have an effect, it could be your neighbor's.

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.