Alternator interference

[Xaccers]

My car radio has a stereo jack socket for aux devices, and I'm currently running my iPaq through it to listen to mp3's and TomTom.
Power from the iPaq is via the cigarette lighter socket, and this is giving an annoying whistle at high revs (over 2000rpm)
When running on the iPaq's battery, there's no whistle.

Any ideas on what I could do to cut down or erradicate this whiste?

[Taf]

Fit a ferrite ring around the power cable (looks like a black doughnut).

http://www.mp.ttu.ee/risto/rc/electronics/images/esc/future35/ferrite.jpg

[Xaccers]

Fit the ring around the cable (maplins do clip on ones that the cable goes through), or wrap the cable around the ring?

[Creative]

Can you run a dedicated power lead from the battery? If so that should also help. CB/Ham radio users don't use the cigarette lighter socket, and run a direct (FUSED!) connection from the battery to the equipment for this very reason.

[Xaccers]

Without hacking the cigarette plug off the end of the power lead, not really.