I bought some "accustical" foam from a company online that makes sound proofing material for buildings, auditoriums, music rooms, etc. They had a clearance section selling production scraps, so I figured
what the heck! This 5x6 1/2 sheet cost me a whopping $20 shipped. I used it in the big open voids to get some sound absorption going on in addition to the barrier action of the LL and the metal dampening of the Damplifier.
For example, I stuffed it in the opening above the rear strut and wheel well where I couldn't put anything else.
I wrapped some of the foam in plastic to protect it from moisture and then put those bags over the wheel wells and in the side behind the front door.
