Check to make sure you don’t have a plug wire in the cap arcing to the windshield wiper motor bolt. I had bad radio noise for about a year and tried filters, wiring directly to the battery and everything else. Since I didn’t listen to the radio much I just left it turned off. One day while setting timing I turned the garage lights off to see the marks better. Happened to look towards the windshield and saw the arc. Repositioned the distributor and replaced the boot and all is well. The arcing didn’t effect the running of the car and if I hadn’t turned off the lights I would have never known.