Home air vents deliver conditioned air from an air handler. The cause of noise that is heard through the air vents is usually the fan in the air handler. For light sleepers or those in a room near the air handler, the noise can be significant. Silence the noise by installing an air-duct silencer. The air-duct silencer will not only stop noise from the air handler but reduce other noises that travel through the air ducts.
Turn off the air conditioner or the furnace connected to the air ducts.
Locate the duct work that leads to the room with the noisy vent. This may require tracing the duct work to the attic of your home.
Cut the air duct in the middle of its run. Use a utility knife to cut through the insulation and the duct. Use tin snips to cut the metal wire on the air duct. If you have a rectangular air duct made of sheet metal you must cut it with the tin snips.
Push the duct silencer into one of the ends of the cut duct work and secure it with tie wraps. If you have rectangular sheet-metal duct work, secure the silencer to the duct work with the drill and sheet-metal screws at each corner of the duct. Connect the other end of the duct to the remaining end of the duct silencer using the same technique.
Seal the seams. If you have round ducts, pull the insulation over the seam and wrap it with at least two layers of duct tape. If you have sheet-metal duct work, wrap the duct tape around the screws and the seam to seal it.
Turn on the air conditioner or furnace and check the room for reduced noise: there should be a significant reduction. If the noise continues, contact a mechanical contractor to take a look at the air handler.
You can buy duct silencers at most HVAC supply houses.