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?

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3 answers 1 comment
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
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!