I really am not sure if there is a simple resistor that could be put in to cure it. I know resistors can be used for noise filters such as "bass blockers" but you would want to know the frequency the noise/hum is at to select the right resistor, and then if it is in the musical range then you will loose that portion of the music as well.
I am guessing this noise is being picked up by RCA cables run under the door sills. If so it might be easier/better to re-route the RCA cables away from the sills and hopefully remove the noise the RCA's are picking up.