What i'd do it with the car off and cold, put yer hand under the exhaust, move shake it at different places slightly and if you hear it hit a heat shield, then you localized the location of contact, just get a breaker bar, wedge it between the exhaust pipe and the shield that it is hitting, and bend the shield back and hold it for like a minute, try shaking the exhaust at the point again. If it don't contact with the same amount of movement, then your car shouldn't rattle anymore, if you make contact repeat proceedure again.

I've done this on my car, it worked with success.