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My gas furnace is in a dedicated "closet" inside the utility room. The closest cold air return is about 5' away (laterally) in the ceiling of the hallway right by the bedrooms. When the furnace lights you can hear significant combustion noise, and when the fan is blowing the air movement is loud, and fast enough to produce a slight whistling through the vent.

The area around the return vent looks like it's insulated, but the vertical duct (~8' long; it's an updraft furnace) gives a good ring when I thump it, so I think it is not.

Is there anything I can do to quiet the vent? Would replacing the return duct with insulated ducting help? I know a multi-speed blower would help, but this furnace is brand new (new construction). Can I install a more restrictive vent and shift more of the return flow to the upstairs return (which doesn't seem to make any noise)?

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If you shift more airflow upstairs, it's likely it will become noisy. Do not restrict anything unless detailed duct size analysis indicate it is indeed oversized. Insulating ducts will reduce resonant noise, and slightly reduce carried noise such as combustion noise, but it will not stop the whistling and air flow noise at the grille. If you do get insulated ducts, they need to be upsized to make up for the insulation thickness.

To reduce whistling and air flow noise right at the grille, you need a less restrictive grille. Sometimes just a different grille of a different configuration will stop whistling, but not air flow noise. If a less restrictive grille is not possible, the only other solution is to increase the size or add another inlet so there is less demand on the one by the bedrooms.

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