Chances are your power supply fan is getting ready to pack it in. The fan inside your computer has the job of keeping your power supply cool while the computer is on.
If you are lucky, the fan might just be clogged up with dust and even pet hair. To check this, you can shine a flash light into the back vent of the computer to see if everything inside is covered with dust. Better yet, don't be afraid to take off the back of the computer and have an actual close up look - but be sure to unplug the power supply.
Many people are intimidated by the idea of taking of the cover to their computer, but it's not much different than opening the hood of your car - and you won't hurt anything.
To clean your fan, use a can of compressed air which you can buy at just about any electronics store or office supply store. Blow the dust of the blades of the fan and off of anything else that is covered.
If the fan still is making noises, it likely means you will need to change the fan. You don't want to let this go too long because your mother board can be seriously damaged from an overheating computer which will pretty well ruin your entire system.
Changing your power supply fan is a bit more complicated though and unless you are technically sound or experienced, you should likely leave this to a technician.
