yeah its bacterial build up inside the heater box. (changing the pollen filter is a good idea too.) we use the forte spray can treatment one, its quite good.
set the car up windows shut, engine running, air con on full cold, full fan, set to recirculate.
stand the spray can up in the front passenger footwell, click it down and it sprays constantly for a couple of minutes. set it going shut the door and leave it for 20 mins. then open all the windows, turn it off, and leave it for a hour before driving.