What is the best aftermarket material I can use for sound deadening the inside of my trunk? I recently installed a truck kit and now I hear the exhaust. Kit came with thinner carpet and some sort of sponge vs jute.
There is some good info on the web.
I have installed Dyna Mat (there are others, read up on it) as the first layer and then some 3/8" closed cell foam (also peel & stick) on top of the Dyna Mat. The butyl rubber mat density stops attenuation from the metal - the foam blocks noise - and they both help insulate a bit. Closed cell foam is way cheaper and helps avoid a 2nd layer of DM.
The DM will glue entirely to the metal, act like a coating of tar or rubber and deaden better than any carpet padding which would be less dense.
ALso, DM is thinner than padding.
I've used Dynamat Extreme on doors, trunk, and package shelf in the past, with good results. I've heard stories of people using adhesive roofing membrane as a cheaper alternative, but I can't speak to any hands-on experience with that.
Deadener will help with resonance, but for actually blocking the sound, you need something like mass loaded vinyl. This guy sells some relevant products, and has a a very good tutorial. https://www.sounddeadenershowdown.com
I haven't used any roofing products in a n automotive environment, but have heard of problems with smell and losing adhesionin high heat conditions.