Does anybody listen to lousy-sounding AM radio anymore?
The frequency response is from 200Hz to about only 2kHz.
The important audio from 2kHz to 15kHz is missing.
The low end of AM audio is not limited to 200 Hz. You get all bass frequencies.
The high end of AM audio is not limited to 2 kHz. The channel is limited to +/-5 kHz, but the chief engineer of CKY AM told me they broadcast up to 15kHz (but I doubt his competence. He's a P. Eng.)
You've been thinking of the limited telephone frequency response.
It is the receivers that limit your high frequency response. Delco car radios are reeeeeally bad for this.
I used to manage a car radio installation facility (16 bays!) My recommendations for the car radio noise:
Check that the antenna ground at the fender has good continuity.
Check that the shield of the antenna cable has continuity from one end to the other.
Check that the car radio chassis is grounded to the frame by way of a metal brace. In some instances, the dash frame might not be grounded to the rest of the chassis, but it should be.
Check that spark plug wire is good and the resistive (carbon) type.
If the noise is a whine, it is the alternator that is generating the noise. If it is a VW, there should be a capacitor mounted to the alternator. It looks just like an old ignition "condenser".
Bob