I see just two possibilities here:
- This is a serious problem with a major component.
- This is s minor issue with some loose external part.
Just in case it's #1, I suggest you start keeping a little black book. Every time you hear it, every thing you to to try to find it, every small problem you find, every communication with the dealer. And yes, you
must communicate with the dealer. Repeatedly. Date and time for all of the above.
That way, if this goes bad, you have some backup for any warranty claim. I have seen it time and time again - a written log beats "he said, she said" every single time. Details are to logs as cubic inches are to horsepower. More is better.
If it rattles when you are not moving,
a mechanic's stethoscope will help a LOT. It's amazing what you can hear with those.
Kind of hard to use it while driving. though. Hmmm. Maybe with an extension tube? For example, rig an esteniton down to the HST so you can listen while driving?