many poeple get an revs related whine after fitting a cd changer or phatbox. after i fitted my phatnoise i had 2 noises:
1) high pitched sound like morse code going on in the background
2) engine speed related quiet whine that many people had suffered.
to get rid of 1) i wrapped the phatbox in a thin layer of plastic sheet and mounted it in the changer holder using nylon screws - this removed the 2nd earth path via the case and worked a treat
to get rid of 2) i tried all sorts including putting multiple ferrites on the changer cable - none of which helped until i also put one right at the end of the cable near the back of the radio - and hurrah, the engine whine has gone completely.
i was considering running separate phono cables to the phatbox for the audio feed to the head but dont need to now at all.