My wife's 2007 Mazda3 2.3L AT has developed a loud noise when shifting from park to reverse. The problem is worst during cold start-up. It is somewhat difficult to shift into reverse, then there is a loud clunk, the car shakes a bit, and there is a persistent rattle (sounds like the radiator fan hitting the shroud, but the fan isn't on). The noise goes away once the car is shifted into neutral or park. Doesn't seem to happen when in drive, although she said she heard the noise yesterday a couple times when turning. After a handful of seconds of the engine running the noise disappears.
A Google search seems to indicate a bad motor mount or maybe CV joint. The car isn't moving, so it seems unlikely that the CV would cause the rattling, but who knows. I checked the passenger-side motor mount, it seems to be intact. No oil beneath it, anyway. It was getting dark out, so I couldn't see the driver's side mounts well.
Any other thoughts on what it might be? What else should I check? The car might still be under warranty (need to check whether the extended warranty was 5 or 6 years), so a trip to the dealer may be in order.