Dynamat or whatever other dynamat-like substance you use adds weight to the particular panel you apply it to therefore reduce it's resonance. If you apply enough of it will stop rattles. And it does reduce some road noise.

But if all you want to do is lower the exhaust and road noise there are much cheaper and easier ways to do so. The cheapest and probably on of the most effective ways to do this would be to go to a carpet store and ask for their remnant scraps of carpet padding. Put it wherever you feel you need to reduce noise the most