Ever opened up the case and cleaned out the inside? A lot of dust collects on the cpu fan, and it could be raising the temp enough that the cooling fan kicks in under circumstances where it wasn't needed before.
I've had my HP for 4 or 5 years, and recently, it seems the fan (or something that sounds like one) is running more often if not almost constantly. Sometimes it runs for 5 minutes, and other times it lasts so long I just shut down my PC. Ideas?