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Noisy motor / knock / vibration

What is going on

Possible causes and checks for: noisy motor / knock / vibration.

Likely causes

  • Belt too tight or slipping
  • Worn/dry bearings (brief noise then seize)
  • Excessive endplay (knock)
  • Bent shaft or debris
  • Rotor hitting stator
  • Misalignment with compressor
  • Wrong air gap
  • Loose fan hub
  • Unbalanced blades
  • Blades touching shroud
  • Worn oilless bushing

Check in this order

  1. Listen at start-up (rumble = bad bearings)
  2. Dial-indicator endplay vs spec
  3. Belt tension gauge and pulley alignment
  4. Tighten setscrew
  5. Replace blade
  6. Reposition fan/housing. Find and fix cause of bearing overheating (oil weight, dirt, tight belt, pulley rubbing, alignment) before fitting the replacement motor

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Source: General HVAC service practice — verify per model. Written in our own words; check the manual for your exact model.

For licensed technicians. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Shut off power and gas before servicing, never bypass a safety control, and follow the manufacturer’s manual for the exact model; it and local codes always take priority. HVACopedia is not affiliated with any manufacturer; brand and model names only identify the equipment.