Wow. I thought I was crazy, but there are at least 2 of us. My 2004 S-Type 3.0, 87k, does the exact same thing and has always done so since I got it at approx. 70k. It occurs in a very narrow range, 32-34 mph; and only when lightly accelerating. (That’s where the hill thing comes in, because you have to be lightly accelerating but not moving out of the 32-34 mph range.) It’s like you’re driving over very weak rumble strips about 20 feet apart, kind of subtle. It hasn’t gotten noticeably worse or different in the 2 years I’ve had the car, but like dhfenton I wish to heck I knew what it is.