It could be the pollen filter - very easy to change (and pretty cheap).
I have also suffered an unpleasant smell from my air vents and blamed a build-up of mould inside the aircon caused by dampness/condensation inside. I used some stuff from Halfords which is meant to sort this problem - it did, but left it's own unique smell. But that faded after a while.
Change the filter as said, but also cut a sliver or two from one of your favourite air freshners (nothing too strong :huh: ) and place them in with the filter. This should help but make sure the pieces of freshner are secure in place too
Edit: Sorry, only realised this thread was dragged up from over a month ago