i'd bet my life-no,make that my wife
-that your rattle is exactly that-your heat shield.the shields are usually just tack welded to the exhaust and over time these welds break.what may seem tight at rest can vibrate once your under way.i've found a screw hose clamp around the shield and exhaust(if you can't get a big enough one try joining acouple together) will do the trick.good luck
