[olddog]

After you have ruled out any external things causing the rattle, you most likely have a baffle loose inside your side pipe. I had the same problem, and someone on here told me to drill a small hole in a place that cannot be seen. Then use a stainless steel sheet metal screw to screw into the baffle and pull it tight to the side pipe.

I use about 1-1/2" to 2" stainless steel sheet metal screw. Likely a #12 screw. I started with a small bit that the screw could start into. I drilled into the side pipe from under the car. After drilling through the side pipe, I then drilled through the baffle. Then I changed to a bit big enough that the screw could slide through, and drilled the outer shell only. This allowed the screw to tighten into the baffle and pull it tight to the wall.

You may want to understand the design inside to know where to drill. I was able to see with a mirror and flashlight. A small camera would be even better.

The down side risk is if the screw ever backs out, it could go into a tire. However the rattle should come back before it backs out. Mine has been this way about 5 or 6 years now.

The only other solution is to cut the pipes open to fix it right, which will require striping and re-coating after welding it back together. That or just replace them. Either way, it will be costly. A couple screws for a couple bucks is worth a try.