Ignoramus1606 wrote:

I did a quick google search and found many types of sonex. Which one
would you use?

You may want to think about this for a minute. You are looking for
transmission loss and vibration damping, properties for which Sonex is
not noted. You want to look at how the noise is being coupled to the
air. The big problem in this type of system is providing the combustion
air while maintaing some acoustical isolation. Some time in the library
may save your buying materials (lead, foam, or whatever)that don't do
the job you had envisioned. You may want to try integrating the sound
isolation properties into the structure instead of tacking up stuff and
hoping for the best.

Kevin Gallimore