[Overdrive]

The rattling is definitely not normal behavior, and like others have said is probably from a loose panel or something else vibrating against something else. The reason you may not hear it when not in a driving gear is because in park or neutral there is no load on the engine. Things behave a little differently when the engine is loaded, whether it be that they move/vibrate a little differently/more, or sound different.

Ever notice how when you go from Park or Neutral into Drive that the rpms drop a little bit as the car goes into gear? That's putting a load on the engine, giving it something to do...in this case move the car forward. Without that load all it has to do is spin, which is pretty effortless. In gear, now it's got more machinery to move, and you'd be surprised how much things you'd expect to be rigid do move under the hood and under the car. Vibrations and frequencies are strange things.

Anyway, point being, something is probably loose, maybe missing a few bolts, like a heat shield or one of the underbody panels, or an exhaust bushing may have fallen off or not even be there, letting the pipes move around a lot more than they should. That's my guess, and hopefully that tech will start with those simple things before trying to deep dive into some exotic possibility.

Also, yes, your automatic car does have a flywheel, it's just usually a slightly different design than what is in a manual car, but still performs the same function. I do not suspect that is your issue, though.

Keep us posted.