[gregcss]

My HVAC system is in the living room behind a slotted door. There are no intakes here, they are else where. I'm looking at 1/2" 'high quality' neoprene foam roll from The Foam Factory (http://www.thefoamfactory.com/closedcellfoam/neoprene.html) to cover the door and possibly line the interior of the door with the same or other foam. I'll likely experiment with lining the interior of the door with cardboard and duct tape first. Any thoughts?

[rajacat]

Try mass loaded vinyl.

 Lot's of useful info. on this site.
http://www.supersoundproofingsales.com/Mass-Loaded-Vinyl-45W-w_PSA-per-foot/productinfo/09-00005-55-PSA/
http://soundproofing.org/infopages/flooring.htm

[mcgsxr]

Any idea why the door is slotted?

If it is not a requirement, I imagine a Safe n Sound door and a good gasket all around the door would do wonders.   Under $100 at HD.  I use one at the top of the basement stairs with a good door sweep to help contains the HT sounds to the basement. 

[gregcss]

Thanks for the idea. This is the door here:

[Ethan Winer]

My HVAC system is in the living room behind a slotted door.

If the noise is mostly low frequencies, foam will do nothing to help that. I agree with the advice to replace the door(s) and seal the edges well. That will also work better than MLV.

--Ethan

[EdRo]

     Hey guys. Just my 2 cents. I played bass in a metal band, in my basement. The ductwork sounded like it was ALIVE!!! After digging out all the loose change and toys it still vibrated like a wild thing, so I rolled contact cement over all the metal ductwork, and stuck on heavy carpet pad. It worked VERY well, and it was something that I could afford.    Now, it wasn't very pretty, but a metal band doesn't care about "pretty"!

[MtnHam]

Any idea why the door is slotted?

Where does the combustion air come from?

[mcgsxr]

I was assuming a high efficiency furnace (like mine) that has a feed directly from outside.

That negates the need for a "vent" of any kind between my finished space, and my utility space.