bacteria etc in vents or something like this... smell usually goes away, or you can get some treatments where you put the system on re-circ and set off a spray inside the care.
OR... who changed the pollen filter? its very common for it to be fitted incorrectly so the cover over the pollen filter doesnt cover it properly. When it rains it gets wet. So the fact a new pollen filter has been fitted could actually be the
problem..