It's critical that we can hear this to diagnose it. Words are rarely any use for sound. For example, one crackle a friend described turned out to be instantly identifiable and therefore curable.
If I have understood properly, when the engine is running you get some kind of interference, or is it when the ignition is on, but it is evident inside and outside the vehicle on two separate and different aerials, and it does not matter if the radio is physically and electrically connected to e vehicle, or working totally independently on its own battery power. We quite like mysteries, but it really is like describing a colour without referring to another one - we all have a different idea in our heads. It is possible that your vehicle computer is putting out loads of hash, it's possible there is a physical problem with the ignition. Coil packs, for example, have suppression built into them but if you have a diesel this can't happen. Do you have a 12v to 240v inverter on board and connected? These can oscillate sometimes. Interference needs accuracy and repeatability to solve. If your vehicle has a fault it would be better to fix it, rather than give up. How about a clip with your phone and slap it on YouTube and give us the link so we could see and hear, and maybe at the very least, cross off some things it cannot be?