Sounds like a ground loop in the signal connection - you need to insulate the ground conductor of the signal cable as though it were a hot conductor. It should connect the ground of the car player's outputs to the grounds of one or more amplifier inputs, and nothing else.

This means if you have another head-unit you cannot join the grounds of its aux out to the ground of the car player's output. OTOH it is fine to connect any old line source to the car player's aux-in since that has a floating ground.

I made this mistake once trying to use a relay to bypass the car player when it was removed from the sled - I switched the signal conductors and just joined up the grounds of the two sources. You still can do this, but you must switch the grounds too using a 4 pole relay.

Cheers, John