are there high frequency filters?

Sounds like a typical ground issue, especially with a computer.  Ground the power supply to the car's battery.  If you're using stock speakers (UGH!) why do yu have a crossover?

If you really want to build a low-pass high-frequency filter, a simple first order system would ba an inductor in series with every speaker.  I suggest trying about 0.04mH (assuming a 4-ohm speaker.)  You may have to wind your own to find one that small, by the way.  Or get a good full-range 1/3 octave graphic equalizer, which will have the added advantage of being infinitely more flexible.

That sounds like ground loop to me.  I was alright, and had no noise, until I added my cheapo lanzar crossover, then I would hear a high pitch whine when I would rev the engine, only from the fronts, so I went to radio shack and bought a couple ground loop isolators.  This really did mask the problem, but I couldn't hear it, and thus I was then satisfied.  I subsiquently I removed the crossover after I opened the trunk after rain and poured water all over it, killing it.  I now use the x-over in the headunit, and am satisfied again.

Kevin

I like trobleshooting.

- does this noise stay steady, or go loud and soft when you drive, turn the AC on, lights, ect?

- Is this noise up front, in the back, or in the trunk?

- how loud is this noise in relation to the music?

- Does this sound get louder as you turn up the volume?

- Do you "hear" the sound or do you just have a mysterious headache when you listen to the music?

thats all I can think of for now.

Kevin