Had the same issue here, and I finally realized there are only two options, other than moving to a different house.
EMI is everywhere, the wiring, the microwave, the cordless phone, the CB'er next door, etc....
All you can do is
(1) learn to stand at the specific angle relative to your monitor where the interference is least (usually with the guitar at 90 degree angles to the monitor)
OR.....
(2) Have noiseless pickups installed on your axe.
I did #1 for years and years, but paying attention to that all the time really sux the spontaneous passion out of a guitar track.
Then I did #2.....and I'll never go back!
Of course, I'm assuming you have already eliminated the power supply as a culprit? And you already have great cords? I even put those little ferrite chokes on all my cords. Now my guitar is hum free!