personal experience, it seems that all tweeters (the higher the frequency) output some kind of engine noise. I have a few fixes.
1.) Reground Batter to frame, (NEGATIVE POST!!!!!!!!)
2.) Reground Engine to frame.
3.) Reground Radio to chassis frame as opposed to the wiring harness, with as thick and as short of a wire as possible.
4.) If you have multiple components hooked up (tv, amp, radar, detector, etc) Go to radio shack and buy a Power Ground Loop Isolator.
5.) If none of these work, just lower the high end of your crossover and cope with it.... or buy new speakers that don't go up so high.....