Even though the Batpod itself is powered by a conventional internal combustion engine, the moviemakers wanted the machine add tension to the chase scenes in two ways. First, Batman is very exposed on the bike, unlike the heavily armored Tumbler, so we're more worried about the hero. Second, by using a Shepard Tone, the director could continually increase the pitch. The team wanted to start with something real before playing around on a computer to create the tone, and the Roadster's electric motor provided part of the underlying tone. Compare the audio clips below (provided to us by Tesla) with the sound in the movie.
If you have access to the two-disc Dark Knight DVD, you can learn more in the "Evolution of the Knight" featurette on the bonus features disc. At about minute 5, the video starts describing the Batpod and, at 9:30, it gets into sound design. There's a brief video of the Tesla recording session at minute 10.
Hear more clips after the jump.