[rattlesnakepete]

I experience what you are describing nearly everytime I drive on the street. I have even captured it on video tape when using stereo microphones in the cockpit of the car. It took me a while to develop a hypothesis but I believe it comes from separation of the two cylinder banks achieved with side pipes, i.e no cross-over tube. To me it is like alternating pressure waves on each ear drum that are not in synch. I would imagine that different firing orders would impart different rythms as well.

I like it myself and like to consider it a good sign that my engine is healthy and happy provided the ryhtm does not change drastically under similar conditions.