I actually removed the grills to take them out of the equation but will do that.Did you also apply silicone adhesive between the woofer grill and cover plate as I mention in my 1st post? ..Those had quite a bit of vibration until the 2 plates were isolated from one another using that adhesive....
I also stuffed quite a bit of foam in the empty cavities within the subwoofer cover itself...
For the subwoofer enclosure, I was sticking my hand in it to see if I could find where it was coming from - will try the foam but it sounds (as best I can tell) like it is coming from the woofers or the plastic boxes the woofers are housed in.
I guess the true test would be to take it apart and just have the woofers in their enclosures.
I'm starting to think that this was not a design flaw, but rather a way for BWM/Toyota to avoid having to spend more on rattle avoidance.
It's amazing how much more bass this mod creates - rattles were not present in the stock config!