I thought it was fairly common knowledge but guys are using flashing repair as sound deadening material in their vehicles as a cheaper alternative to Dynamat etc.
I know what you're talking about now, lol.
It is a roof flashing material that they sell at Home Depot and other similar stores. While I don't use the Peel and seal brand, I use a product that is identical. It's easy to work with and really does a good job at insulating/sound deadening. I use a small propane torch to apply it and it really is easy to do. The stuff really holds up well too! One word of caution... Don't try to remove the stuff. It's permanent in my eyes as it is a real mess if you ever need to take it off.