First Legacy post!
Congrats.
My car(Forester) also has those rattles. As far as I can figure out, it's on the downpipes.....
I managed to tighten up the clamps around the CATs by inserting left-over internal "padding" from when I broke out the CAT cores. You could use S/Steel scouring pads (similar to "Goldie Locks")
The problem with the shields around the downpipes is that the bolts are so burnt (corroded), if you try to loosen them, you just bend/tear the shields.
I tried pumping in some silicone around the shield clamps, but that only lasted a very short while.
I suppose, having removed the CATs, these shields no longer serve any purpose?
So, if you find a solution to this, please share.