Dynamat 10425 Xtreme Wedge 1 Sheet

This has a lot of uses beyond car applications. I used a sheet of it on the drywall behind in-wall speakers to cut the amount of sound radiating through the back wall and tighten the sound. I used a sheet of it inside a computer case, to eliminate a tinny rattle that came from the case vibration while it was on. You can even put pieces of it on the inside of washing machine or dryer panels to make them quieter.

It does have one drawback, and that's the fact that once you stick it on something, you will not be able to get it off without making a big mess. It is essentially tar with a foil outer cover, and trying to get it off just leaves a bunch of sticky tar behind. So, choose the place you want it carefully before sticking it down!

This product is made for automotive uses, but I use it to quiet down my room air conditioners. Most of the newer units are made with very efficient styrofoam insulation that does not deaden sound very well. Applying dynamat to the barrier between the inside and outside of the unit makes a huge difference. It ought to be great for your car!