@helenkj sounds like a bearing issue. I would suggest you replace the fan. It's a hit and miss with other makes as well. It all depends on "the mileage" of the fan and your climate. The harder it has to work the sooner it will wear out.
Adding oils or even WD 40 will at best only work for a brief amount of time. If you are already there to oil it might as well go all the way and replace it. Download teh maintenance manual for your computer [http://h10032.www1.hp.com/ctg/Manual/c05485525.pdf|from here] to help you with that