Its probaly getting noisy because its working to hard. The RPMs are increasing and thats probaly the noise you are hearing. I recommend cleaning your CPU with one of those compressed air cans. Get all the dust out of your computer. Then check how many programs you are running at a time. I would start at that first.
I agree with Berny, the number one cause of fan failure is due to either dust on the blades or dust clogging the fan filter or vents. But the RPM's aren't increasing, they are decreasing. The plastic fans that almost all computers use have very lightweight blades and accumulating dust adds strain to the motor and also can throw the fan out of balance which leads to noise and eventually, failure.
You probably have an 80mm fan and they only cost a few bucks and are very easy to swap out.
They are easy to replace. This reminds me of a PC I had to fix a while back..... it was on the floor in a very dusty area. The power supply fan was seized with dirt..... I could turn it but it was too stiff to turn on its own. When I took the power supply apart there were several exploded components (must have overheated without the fan). And it also took out the motherboard when it blew........
I'll get some of that compressed air, open up the case and blow things out, hopefully that will do it. Its just that a couple of times it has got really loud, and once, when I first started it up, it showed a screen saying the fan wasn't working, but then I shutdown and restarted and everything worked fine...well at least if I do have to replace it, it will be cheap and easy, hell I havent even had the computer for a year,but we do leave it on all the time.
Yeah I see that alot MS. When a PS fries and takes out other components in the chain including the mobo, it is almost always a cheap, stock PS that doesn't have beefy rails (especially the +12V rail) that can handle a surge or short circuit. What happens on a low quality PS is that the rails can't handle excess power due to a low amperage rating, and this cause the rails to overload and damage capacitors and other internal components, and in turn sends the voltage on down the line. This is why one of the most important parts of a pc is a high quality PS like Thermaltake, Hiper, Ultra, Enermax etc.
Compressed air won't do much to the fan blades Wolfman. Get like 10-12 Qtips, wet one end of all of them with water or alcohol and clean both sides of each fan blade until all the dust/silt is gone and then flip the Qtip over to the dry side and wipe the blade dry. And make sure to remove the fan filter, if it has one, and wash it out in a sink and also vacuum the holes or slots where the air escapes to the outside of the case.