TL;DR
Metallic noise from a rooftop unit is usually a loose or bent fan blade. Check the condenser fan blades and set screws before suspecting the compressor.
What you might see
- grinding or rattling from the rooftop unit
- noise varies with fan speed
- metal-on-metal clanking inside the unit
- vibration felt in the ductwork below the unit
Likely causes
Loose or bent condenser fan blade striking the fan ring or internal ductwork
Fan blade set screw loose, allowing the blade assembly to wobble on the motor shaft
Failing compressor bearings producing a grinding sound at start and during operation
Loose sheet metal panels or fasteners vibrating at fan speed
Required tools
- Torque driver (set screw tightening)
- Wrench set
- LOTO kit
- Replacement fan blade (if bent beyond correction)
Safety first
- Lock out the unit before opening the fan section access panel. The condenser fan can start automatically on high head pressure.
- Rooftop work requires fall protection per site requirements.
Procedure
- 1
Lock out the unit at the disconnect before opening any access panel.
- 2
Inspect the condenser fan blades visually. Look for bent blades, missing blade tips, or deposits that unbalance the assembly.[1]
- 3
Check the fan blade hub set screw or retaining bolt. Loose set screws allow the blade to shift and strike the fan ring.[1]
- 4
Spin the condenser fan motor by hand. Rough rotation or grinding indicates failed motor bearings.
- 5
Check all access panel screws and cabinet fasteners. Tighten any loose fasteners.
- 6
If noise is from the compressor compartment (separate from the fan), listen for the character: liquid slugging sounds like a heavy thump on start; bearing noise is a continuous grind. Liquid slugging requires refrigerant charge and TXV investigation.
- 7
A compressor with confirmed internal bearing noise must be replaced. Field repair is not possible.
Sources
Trane Voyager Commercial Rooftop Service Literature, Trane (Trane Technologies)
Trane Voyager Commercial Rooftop IOM, fan assembly and compressor noise diagnostic procedures (general)
View source
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