could be other things but ide say pollen filter, defanitly be the first thing ide check as its a 2 minuit job to check. and if it does need replacing its not very expensive. if its not that it could be things like door seals ect.
if your not sure on how to check the pollen filter have a search for how to replace the pollen filter, but just use that as a guide of how to get the polen filter out and have a look at the filter and the cover/ housing for filter just to make sure theres no cracks anywere and have a look that the cover is compleatly on before you take the filter out, should be able to check tomorow before you set off for work/college/what ever it is you do/ when ever you have 5 minits spare.