A poor audio coupling or ground can cause the hum as well.
Maybe we should DF the interference just for the fun of it. It's not too hard to do. While mobile, measure the signal strength in your area. When you have a full scale signal remove the antenna or change from a VHF to a shorter whip to reduce the signal levels. When that gets full scale remove the antenna open the squelch and hunt again. Also the signal may be coming from an EMS station or a hospital, so start concentrating hunting in those immediate areas.
Drive safely, you do not want to get nailed for distracted driving or worse, smash up your vehicle.