Do you hear everything being said down stairs through your heating ducts? Here's What to do:
1. Follow the instructions for noisy plumbing pipes the difference being that it is in the ceiling rather than a wall. You will be cutting along the floor joist rather than studs. Identify your duct cavity (between two joists) by measuring from your heat register above. Measure out from the wall 3' and make a mark, continue and make another mark at 7'. This will form a rectangle on the ceiling 4' by the joist cavity width. Using a straight edge or level join all your marks.
2. Cut along the lines, press firmly as you are cutting through 1/2" or 5/8" drywall or plaster (if plaster a drywall saw may be required).
3. Remove piece exposing the duct and save, you will be able to replace it after.
4. Using metal plumbing strap support duct at ends of the hole by screwing into joist and using a metal screw into the duct.
5. Remove 3' of metal duct using tin snips and hack saw blade utility saw.
6. Now you're ready to install a duct muffler 5" x 40". If not available in your area at heating and ventilation stores you can make one.
7. Purchase piece of furnace hot air plenum insulation available in sheets (approx. 1" thick) 3 feet by 4 feet (normally the minimum that is sold)
8. Measure circumference of heat duct and cut required size of hot air plenum insulation.
9. Using a drywall knife and straight edge cut vees every 1" along the length of insulation. Vee's should be approx. 1/4" open to allow hot air plenum insulation to form around duct when rolled.
10. Roll the insulation and secure the seam with duct tape.
11. Insert muffler over cut ends of duct allowing 2" overlap at either end.
12. Wrap joints with duct tape fixing your duct muffler in place.
13. This will now greatly reduce any sound being telegraphed through the duct system.