Musty smell from chiller AC vents — what causes it?

Had exactly this. The technician showed me the drain pan and it was… grim. Cleaned out and disinfected, smell gone the same day.

If the musty smell seems to come from the vents/ducting rather than just the unit, it's worth checking whether the ducts are sweating — there's a newer thread on AC ducts condensing and dripping in the ceiling where failed duct insulation lets the cold surface sweat in the humid ceiling void, which then breeds the mould you smell through the vents. Different cause from the coil/drain biofilm, and the fix is above the ceiling.

Adding the seasonal piece from the FM side: the reason this smell gets worse in peak summer is relative humidity. The more humid the air, the longer the FCU coil and drain pan stay damp, and warm + damp is exactly what the biofilm and mould feed on — so a coil that was borderline in winter turns smelly in August. It's why a clean-and-disinfect done before summer holds up better than one done mid-season: you clear the growth before the humidity feeds it. If yours is already smelly, get the coil and drain done now, but the durable habit is timing the service ahead of the humid months rather than chasing the smell after it appears.