In the stereo business they call that engine noise. It could be that there is a bad ground. If you wired a custom Stereo in, with Amps, you have a couple of choices, either run the power cables down one side of the car, and the RCA's down the other side.
You can use an inline noise filter, but it can cut the signal slightly.
I would check your radio ground/antenna ground/amplifier ground. and all connections.
If you do have a custom stereo with amps, and you ran the power and RCA's downt he same side, then definately move the power wires to the other side of the car.
I have also heard that some spark plugs can cause this engine noise. if it is definately coming through the speakers, then I would look into the stereo wiring.
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