Most of it is not a fault
This Old House names two causes, and neither is a broken part:
- Proximity. The closer a register is to the motor and the inner workings, the louder it will be.
- Air velocity. Fast-moving air creates the whistling from registers and the vibration in ductwork, and it gets worse when the vents are restricted.
Our reading, marked as ours, and it is the point of this page. A noise that has always been there, or that got worse after someone closed vents in unused rooms, is an airflow problem. A noise that is new and mechanical, squealing, grinding or clunking, is a part. The first costs tens of dollars to address, the second hundreds, and telling them apart is free.
What the fixes are, cheapest first
All from This Old House:
| Fix | What it does |
|---|---|
| Adjust the air handler speed | the expert's own first suggestion, slower air is quieter air |
| Open the registers you closed | closing one affects the whole system |
| Swap a wooden vent cover for a thin metal one | less restriction, less velocity |
| Fit a larger register, for example 6x10 instead of 4x10 | more area for the same air |
| Adjust the balance damper | restricts flow further back, where you cannot hear it |
| Add flexible, insulated ducting between fan and vents | distance and absorption |
| Add a bend to a straight duct run | even a curve helps |
Two of those deserve the numbers This Old House attaches to them.
On register type: a wooden 4x10 register is roughly 50% restrictive, while a metal 4x10 is about 70% open. That is a large difference for a part that costs very little, and it is the easiest thing on the list to change yourself.
On what the work achieves: the guide's expert measured a job where added return ducting and a couple of bends took the noise from 80 down to 70 on the dBA scale, and explains what that means:
And it might not sound like much, going from 80 to 70 on the dBA scale, but that represents twice the loudness. So I'll cut it in half by going from 80 to 70.
Our note: ten decibels looks like a small number and is not. The scale is logarithmic, which is why a change that reads as small on paper is the difference between a system you notice and one you do not.
The one that catches people out
This Old House's expert makes a point most homeowners have never been told:
closing down one register impacts the entire system
Closing vents in unused rooms feels like saving money. What it actually does is push the same volume of air through fewer openings, which raises velocity everywhere else, which is the exact cause of the whistling the guide is trying to fix.
Our reading: if your system got noisier and you can remember closing a vent, open it and listen again before buying anything. Our AC not cooling page records a related finding from the same publisher: airflow in a room measured dramatically differently depending on whether the door was open or closed.
When the noise is a fault
Fixr's furnace repair guide lists the sounds that are not airflow and need looking at: squealing, whistling, groaning or clunking. If that is what you have, the prices are these:
| Part | Cost |
|---|---|
| Blower belt | $5 to $100, part only |
| Blower bearing | $20 to $150, part only |
| Blower shaft | $50 to $200, part only |
| Blower pulley | $50 to $250, part only |
| Blower housing | $100 to $300, part only |
| Blower wheel, the squirrel cage | $100 to $750, including labor |
| Blower motor repair | $200 to $900 |
| Blower motor replacement, AC side | $300 to $2,000 |
| Blower motor replacement, furnace side | $562 to $1,045 |
The part figures are Bob Vila's, the repair and replacement figures Fixr's. Bob Vila puts the labor to fit a blower motor at $150 to $410 for two or three hours of work.
Our reading, marked as ours. The top of that table is a motor and the bottom is a belt, and they produce similar complaints. A belt at $5 to $100 and a bearing at $20 to $150 are the cheap mechanical causes of a squeal, and both sit far below the motor. Our blower motor page works through why the diagnosis is the whole decision on this part, and notes that the same blower serves your heating and your cooling, so a noise in July is the part your furnace will use in January.
Two sounds are not in this category at all. Fixr's furnace guide lists a carbon monoxide alarm and fuel or burning smells among the signs that need immediate attention, and those are not noise problems to be quietened.
Before you pay anyone
- Open every register you closed, then listen again.
- Look at what the vents are made of. Wood at 50% restrictive against metal at 70% open is a cheap, immediate change.
- Ask whether the air handler can simply be slowed. It is the first thing This Old House's expert suggests and it costs nothing.
- Describe the sound precisely when you call. Whistling points at air, squealing and clunking point at the blower, and the two have an order of magnitude between them.
- Treat a new grinding noise as urgent rather than annoying. At $200 to $900 a blower repair is cheaper than what follows if it seizes.
Sources
- This Old House, Common Cause for Noisy HVAC, updated May 29, 2026 (the two causes, the decibel measurement and what it means, register restriction figures, balance dampers, flexible ducting, bends, and air handler speed)
- Fixr, AC Repair Cost, updated January 29, 2026 (blower motor repair at $200 to $900 and replacement at $300 to $2,000, and the note that blower work is one of the longer jobs)
- Fixr, Furnace Repair Cost, updated August 29, 2025 (the furnace blower motor at $562 to $1,045, and the list of sounds that need investigating)
- Bob Vila, How Much Does Furnace Blower Motor Replacement Cost?, updated November 28, 2022 (the blower parts table, belts, bearings, shafts, pulleys, housings and wheels, and the labor figure)
Price figures are national US ranges from the sources above as of the date shown. Your local price depends on labor rates, permits and site conditions. Read how we get our numbers.