New Block Outdoor Unit Hums: Fan Guard Resonating in Wind

A low humming from an outdoor unit at an exposed waterway-facing block, present on windy days and absent on still ones. The unit is not always running when it happens. Cooling is entirely normal.

The single most useful test here is free: switch the unit off during a windy spell and listen carefully. A noise that continues with everything stopped cannot be produced by any moving part, which eliminates the motor, the blade and the compressor together in one step. What remains is something being excited by moving air, and a large flat grille across an open face is the obvious candidate for that.

Air flowing across a thin panel makes it vibrate, and where the panel is held loosely enough to move against its fixings, that vibration becomes an audible note. An exposed position facing open water supplies steady wind for long periods at a time, so the effect is sustained rather than momentary. The machine plays no part in any of it. It is simply the structure that the wind happens to be blowing across.

The guard is secured firmly and its clips cushioned so wind cannot set it moving, which ends the hum for good. What is worth taking away is the test rather than the repair. Switch it off: any noise that survives that is not being made by the equipment at all, and the search then moves to whatever the weather is acting on instead.