Could very well be hard drive failure in early stages... could quit at any moment or go on for another year. Could be a ball of lint in or near your cpu fan or other fan, or it could simply be a fan that is wearing out or is out of balance from dirt, dust lint, or a coating of oily dust.
Take a look at cleaning the system cpu fan, and cooling fans with a can of denatured alcohol, soft cleaning pads, and qtips... as a start.
Get yourself a can of a computer cleaning gas such as Dust Off or CleanSafe Dust Remover that is treated with difluoroethane gas or other gas that prevents the build up of static electricity... clean out all areas with dust... a little bit at a time... Too fast and the canned air freezes up.
There is no real test for a failing hard drive except age... Look at the date code on the label of your hard drive... if it is three years old, it may be aging... If it is five years old, replace it now while you can.
You can also look at the Device manager for any red, yellow, or grey flags... Start->Control Panel->System->Hardware->Device Manager... Look at the hard drive indicator to see if it is SMART OK, or if it has a yellow warning indicator that tells you the fan is going.