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Noise in HVAC Duct Systems: Causes, Diagnosis, and Solutions

HVAC Duct Calculator Team ·

Why HVAC Ducts Make Noise

All ductwork produces some sound as air moves through it. The goal is to keep that sound below the threshold of human perception in occupied spaces. When ducts are noisy, it almost always points to an airflow problem.

Types of Duct Noise

Rushing/Whooshing Sound

Cause: Air velocity too high (above 900 FPM in residential) Location: Usually heard at supply registers or along duct runs Fix: Increase duct size to reduce velocity

Whistling

Cause: Air squeezing through a narrow gap or restriction Location: At partially closed dampers, register louvers, or flex duct kinks Fix: Open dampers fully, unkink flex duct, increase restriction opening

Popping/Banging

Cause: Sheet metal duct walls flexing from pressure changes (called oil canning) Location: Large flat rectangular duct sections Fix: Add cross breaks or stiffeners to duct panels

Rattling/Vibrating

Cause: Loose connections, unsupported duct sections, or duct touching building structure Location: At joints, hangers, or where ducts pass through walls Fix: Tighten connections, add supports, isolate duct from structure

Humming/Droning

Cause: Blower motor or fan vibration transmitted through the duct Location: Near the air handler Fix: Add flex connector between air handler and first duct section

The Velocity Rule for Quiet Ducts

Noise is directly proportional to velocity. The higher the velocity, the louder the noise:

Velocity (FPM)Noise LevelSuitable For
Under 500Very quietRecording studios, hospitals
500 to 700QuietBedrooms, offices
700 to 900AcceptableLiving rooms, kitchens
900 to 1,200NoticeableCommercial spaces
Above 1,200LoudIndustrial, mechanical rooms

How to Diagnose Duct Noise

Step 1: Identify the Location

Walk through the building with the system running. Note where noise is loudest. Is it at specific registers, along duct runs, or near the air handler?

Step 2: Identify the Type

Is it rushing, whistling, banging, or rattling? Each type has different causes and solutions.

Step 3: Check Velocity

Calculate the velocity in the noisy duct:

  1. Measure the duct size
  2. Estimate the CFM (use room square footage × 1)
  3. Calculate: Velocity = CFM / Duct Area
  4. Compare to the noise thresholds above

Step 4: Inspect Physically

Look for:

  • Kinked or compressed flex duct
  • Disconnected duct joints
  • Missing screws or tape at connections
  • Ducts touching framing members
  • Dampers or obstructions inside ducts

Solutions by Noise Type

For High Velocity Noise (most common)

  1. Increase duct size by one or two standard increments
  2. Add a parallel run to split the airflow between two ducts
  3. Reduce CFM if the room is receiving more air than needed (close balancing damper partially)

For Whistle Noise

  1. Open all dampers fully first
  2. Check register sizing (undersized registers whistle)
  3. Inspect flex duct for compression, kinks, or tight bends

For Banging/Oil Canning

  1. Add cross breaks (diagonal creases) to large flat duct panels
  2. Install stiffening channels on long duct runs
  3. Add internal duct liner (also reduces airflow noise)

For Vibration/Rattle

  1. Tighten all mechanical connections
  2. Add vibration isolators between equipment and ducts
  3. Install flexible canvas connectors at the air handler
  4. Support ducts properly with hangers every 4 feet

Prevention: Design for Quiet

The best approach is to prevent noise at the design stage:

  • Size all residential supply ducts for under 900 FPM
  • Size bedroom and office ducts for under 700 FPM
  • Use smooth radius elbows instead of sharp mitered turns
  • Keep flex duct runs short and fully extended
  • Use our duct calculator to verify velocity before installation
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