you have a ground loop, use a ground loop isolator to get rid of it.

It might be the alternator noise. If you step on the gas is it getting louder and more high pitch? If it is, then yes you have weak ground, or so called ground loop. I am having a same problem even after I installed amplifier. I guess this car is not really good.

Ground loop isolators are used to break the DC circuit path for the audio shield ground circuit. It removes ground loops from the system, which are unwanted.

A depression in the ground.

The most common cause of noise in your AM radio is a loose or corroded ground wire. Clean and tighten the ground wire and the AM noise should disappear.