Hi Dan...welcome to the Forum
Usually, the musty smell is the bacteria in the heating & ventilation system and it needs eradicating by using an 'air cond bomb' which is like a smoke grenade which you put the air con on full (recirculation), windows up and let it rip. The smoke/gas works it way through the pipes and tubes killing all bacteria and hey presto, smells nice again
Cheers, Trevor