theoretical about separat pwr cable/RCA

Nice try, but no banana. The noise you are referring to can be caused by numerous problems in vehicles, be it a bad power supply or ground in a deck or amp, no ground strap from a hood, poor battery to chassis ground, poor ground from equipment to chassis and the list goes on. One thing that most of these whining noises have in common is a grounding problem of some type. At one point in time I could guarantee noise in certain model year Toyota's when Pioneer cd players or tape decks were used, yet no noise at all when Alpine heads were used. Do you think it was the vehicle or the design of the power supply in the Pioneer head? A bad voltage regulator or leaky diode can also cause noise headache's, but nonetheless, you may have also stumbled onto yet another cause and posted up for all to see.

I'll have to agree with forbidden, with the grounds, but you "profuse007" also stumbled upon a device that does cut out noise, but just that isn't going to stop all engine noise.  Like you said.

I once had engine noise, then i ran about about 10 grounding wires from different componets on my engine with 8 guage wire, and ran all te wires to 1 grounding location, and voila, my noise was gone, and i noticed a little power gain, wtf, eh, haha now i see people selling kits on e-bay for $20 for doing exactly what i did, for PERFORMANCE gains, lol, , , i stumbled upon a new concept 2 years ago, to bad i didn't market it then, errrrr