I would think an electrician could tell if you had a grounding issue in your house wiring or not. If all the outlets in your room are on the same circuit (likely, but not certain), and that wiring is "correct," then grounding problems are the fault of whatever you're plugging in to it. So, process of elimination to figure out what pieces or specific connections are generating the noise. I.e., start with the interface and computer plugged in and record "nothing" to confirm at least that much is clean. Add one by one, and when some noise enters, change the order of pieces to see if it's a specific combination or just the last thing you added. Etc.
Have a [voltage regulating] UPS for the computer and external drives, and a Carvin conditioner for everything else. All my noise come from the AC inside our house, and the planes and lawnmowers outside.
