It can be hard to diagnose rattling sounds which only occur under specific conditions! Honestly, it's hard to picture the sound based on your description. But, I would throw out there, a rattling power steering pump!
Rattly power steering pump.
I've had what I believe to be this rattle on my 2001 2.7 for a while, and it has a "fluttering" characteristic to it. It's been really hard to capture the sound on a recording, by the way, because it's most pronounced under certain load range, certain rpm areas (and I've noticed it will "come and go" about 3-4 times while accelerating from 2000-3000 rpm, so definitely some harmonic action), and also is most audible only when driving the car next to a sound reflecting wall (e.g., concrete median or wall) on the passenger side. Oh and by the way, best way to describe it is sounding like a metallic rattling noise. So, if your sounds seems to match any or all of those characteristics, check out if your power steering pump has any rotational slop or play in it. I'm pretty excited to have finally figured it out in my car - went through a lot of tests to finally get at it. Best of luck!