Rattling noise coming from underneath the car. on 1998 Volkswagen Jetta

As soon as I put the car in drive and especially when I accelerate, I hear a rattling noise coming from the middle of the car. The noise cannot be heard from outside of the car, only inside. I have been trying to find online a diagram of the underside of my car but so far, no luck. I think my muffler may be loose and is banging into the bottom of the car but I'm not sure. Any ideas?

Asked by for the 1998 Volkswagen Jetta
Typically this is the Catalytic Converter, especially on VW. It usually sounds a lot like rocks in a can. Ring a bell?

here are some shops http://repairpal.com/directory?address=19031&car_brand_names=Volkswagen
3 answers 1 comment
could be those heat sheilds, one covering the exhaust mainifold, or could be some underneath the car.
I have replaced my cat and down pipe and if anything this noise is worse!!! Any other ideas?

I ended up having to replace my entire exhaust system. My muffler was "tin-canning" and the rest was so rusted out that it had to be replaced. Thanks for helping!