Office Ducted Unit Whistles: Duct Clamp Loose at High Speed

A whistle only on the highest fan setting, completely absent on every lower one, across the whole working day. Cooling is normal throughout. The ductwork was last opened during a minor office layout change some time back.

A fan fault produces noise across most of its range, because the fan is turning at every setting. A gap in the duct produces nothing at all until air moves fast enough to whistle through it. That makes the threshold itself the diagnosis: the speed at which the sound appears is the speed at which volume crosses what the gap can pass quietly.

A clamp that is not fully refastened leaves a gap too small to matter at low flow, and air passes it quietly. Raise the volume and the same gap forces air through fast enough to whistle, which is why the sound has a hard edge rather than growing gradually. The disturbance that caused it was a layout change, and a clamp is exactly the sort of thing such work touches without anyone writing it down.

Refasten the clamp and test every speed rather than only the one that whistled, because a gap partly closed is quiet at the old threshold and noisy at a slightly higher one. No internal fan work follows from this. The step worth adding permanently is a clamp check at the end of any ceiling or layout work, since that is when they get disturbed and nobody is looking.