I have an air 'recycle' vent that is prob 3 feet by 3 feet. when heat or ac goes on, the fan in the furnace sucks air thru a filter, into this "box" which is lined with sheet metal. At the back end of the "box", is the furnace. The noise of the air passing thru the "box", is terrible. I cannot lower the furnace fan speed (fan isn't causing the noise...air passing thru, is) so want to sound proof the "box". Since air is recycled thru the house, i'm worried about what i may use, or how i can quiet the system down. Any help appreciated!!!

 

hi if you go to any large supply store you can get some insulation called "duct board" it is used to make those air returnes to reduce noise in the air passage "plenum"
you can duck tape it over your metal one to reduce the noise level

 

Sometimes the grill itself is the problem. Try to bend the slats a little wider or take off the grill and see if the noise persists. If it goes away, try a different grill.