[shorod]

Definitely need to know if the radio is original or aftermarket. If original, do you hear the engine noise on both AM and FM, or AM only? If AM only, you probably have a bad ground, possibly for the hood.

If aftermarket, then check your power and ground connections. If crimp splices were used, I'd suggest cutting them out and using solder instead. As TaurusKing suggested, most aftermarket radios of reasonable quality will include a noise choke (inductor) and filter (capacitor) to filter any power line noise. If the aftermarket radio is a cheapy, then you may need to live with the noise or get a better quality unit.

-Rod