If these shops really did take a decent look at the heat shields, which is a big IF, then another possibility is somethong loose and broken inside the cat itself. A cat with broken insides can make a similar noise to heat shields.

Now all that said, when I bought my car it had such a nasty rattle, which the PO owner had a shop tell him it was a cat with busted insides (car had a cheapo aftermarket cat welded onto stock exhaust, which is a shame since car had less than 80k miles so they could have gotten a free cat from Honda). Anyway, I got under there to confirm it wasn't just a heat shield, and they all seemed to check out. That is until I could really get under there and found it was just the heat shield on the top side of cat. Couldn't find it until I got car high enough to really get under there good. A shop with a lift should have found it, but only if they really made a decent effort to look.

 

It's not your rear view mirror is it ?

 

Sounds like the honeycomb inside the cat. Mine does it at idle and goes away above like 1000rpm. I would imagine its possible for it to rattle at other rpm ranges as well.