Jack the car up and put it on jack stands in the back. Get on your back and get under the car, and
bring a hammer with you.
The metal heat shields on the cats make the exact sound you all are describing.
Hit the living crap out of the loose heat shield to bend it a little and tighten it up. This will feel really good if you are as pissed off as I was with that rattle buzz noises at low speeds in low gears in alley ways or by fencing.
Then get the car back out on the road near a fence and you won't hear it any longer. You will feel the satisfaction of knowing you beat your Porsche with a hammer and that's exactly what it needed and what your mechanic would have done, but charged you for it.
