Dynamat is good stuff. But another option is to find a industrial/commercial insulation shop and ask them if they will sell you a couple of sheets of Armstrong (Armoflex rubber insulation). They come in 4' by 8' sheets and thickness of half inch up to one and a half inch. Not only will it dampen the noise, but the insulation of the heat and cold are unmatched in you auto application. The best part it should be cheaper than the dynamat and ask for a pint can (which is more than enough) of Armstrong 520 adhesive. Use cheap throw away brush to apply glue to insulation and surface to be applied to. Only a one to two inch circle in different spots is all thats needed. Dont glue whole thing, not necessary. It bends and twist easily and will lay flat for easy installation.
I have seen whole vans and vehicles insulated with the foam (Ureathane). Its is so comfortable as far as the A/C and heater go but the biggest problem is the noise and squeakin from the movement of the metal. I would never put any of the spray foam in any cracks or creavess because of sound issues, in any of my vehicles.
Just my two cents worth, and working with insulation for 27 years.
1989 420SEL 105575 miles
1990 Chevrolet E-Cab C-1500
1989 Olds Toranado Trofeo'