It very well may be either a fungal or bacterial growth in the A/C, especially if the odour developed over time. Sometimes it smells bad, sometimes - not, e.g.

It very well may be either a fungal or bacterial growth in the A/C, especially if the odour developed over time. Sometimes it smells bad, sometimes - not, e.g. like a musk. The service can flush and clean the A/C and it's not really expensive as it's a common-ish problem.