I had this on my old 97 S70 T5, it turned out to be the blower motor. DEAD easy to replace via the glovebox and lower tray. The dealer did mine, but it was £300, which i refused to pay because I didnt give them authorisation

Theres some on Fleebay for about £20.00

As said before, also check the pollen filter, it could be WELL grotty

Also, my lights flash too, but my fan still works fine. My aircon isnt that cold, so Im guessing I just need a regas