I had owned two different 8th gen 4-cylinders that had this very same problem. It was very annoying. It seemed the origin was vibration from the engine which caused something in the rear of the car to resonate. Letting off the brake or putting in neutral would make it go away. My 9th gen does not have this issue, no matter what the electrical load is, and it idles just under 1000 RPMs while in drive after being full warmed up. Must be luck of the draw.