All amps have a noise floor, but it sounds like you're describing something other than noise floor. You should not be able to hear the noise floor of your typical amp when standing outside the car.

Most likely what you're hearing is a ground loop.

A typical amp's noise floor is going to sound like a very very faint hiss, like wind through trees. Noise floor will not be "colored" at all, it will just be a dry hiss that is steady and does not change. A ground loop, on the other hand, will often have a character to it, like a pitch or a note, and may change over time.