I hear a bumping sound every time when driving off. It happens at about 10 feet from where I started, and is more noticeable on a bump/dip. It dips coming out of the driveway, so I always here it there. I'm sure it's just the suspension settling in after being automatically adjusted at startup. It seems that it's part of the computer's routine at startup to evaluate the suspension height and make adjustments, even if the adjustments are minor. If you go from one trip having one passenger, and another trip with two or more, the adjustments are quite noticeable. After driving alone, have four adults get in, and it will raise the vehicle substantially.
That being said, there is a theory that in certain places, objects and/or materials have the ability to record past events and play them back at a later date (used as an explanation for ghosts and strange noises). Perhaps in the past someone farted loudly in the passenger seat?