Got one of those portable 40CH CB's, or a 3MHz - 30MHz SW radio, that you could walk around with? This could be anything, (And I've heard that noise as well.), like Panasonic TV's, CATV, or DSL, MODEM, switching power supply for any appliance, computer desktop, laptop, pad, and their associated power supplies, printer, FAX machine, (Yes, I still have one.), power company's newer smart meter, any appliance with "Smart" tech and wi-fi in it, anything! At my QTH, I've used a portable to locate RFI spurs from the DSL MODEM, and our in-ground Clear Stream UV Light Power Supply. (I fixed that one by installing a .1uF - 250V CAP across the DC terminals.) Now, when ya find it, and prove it by powering a device down, then back ON again, see what can be done, like ferrite chokes on power and signal cables, HV CAP's across terminals, aluminum foil tape wrapped around a device. (Don't do it to a wi-fi device with built-in antenna.) Good luck!!!!!! Happy hunting!!!!