I have been under the car and can't find any loose heat shields, or any thing else loose for that matter. I only happens when the car is hot, after a sprightly drive. I would have thought loose heats shields would be constantly loose!!
Mmmm...are you sure it's not just the cats "popping" when they are hot? They will make metallic noises after you drive the car and let it idle or sit which is completely normal. If you are familiar with that noise and that is not it, what does it sound like exactly?