Begin with what is safe to inspect
Turn the system off and inspect the removable filter according to the manufacturer instructions. Look through the accessible opening for visible dust or dark spotting without dismantling the head.
Notice the pattern
Does the odour only appear at startup? Does it return quickly after the filter is cleaned? Is there weak airflow, visible buildup or water where it should not be? The pattern helps a technician understand the condition found.
Keep the claim practical
Moisture can support mould growth on indoor surfaces. The useful action is to address visible buildup and moisture pathways, rather than trying to identify a health cause from a smell.
The practical takeaway
The right response to a musty smell is a calm inspection and a clear next step, not a scare campaign.