Last night I parked the Jeep ('05 Wrangler, 4.0 Inline 6, OEM EFI) at a friends driveway, slight uphill slope. I parked it in either low or second, and didn't set the brake. When I came out about four hours later the Jeep had rolled backwards about ten feet, which would have rotated the engine backwards several revolutions. When I started the engine it knocked something fierce - sounded like every lifter was drained back. I shut it off, re-started and it was the same, but seemed to lessen with RPM's a little. Drove home, about six miles, and it was idling quietly when I pulled into the garage, fully warmed up. This morning it was fine, starting quickly and idling normally.

I'm scratching my head why a few rotations backwards might cause this. Any thoughts, ideas or experience to explain it? I'm not worried about it, but wondering about cause & effect.