Your computer has automatic fan control features. Some motherboards (and PSUs) include this feature. It means that when the CPU (or case or PSU) is cooler, because of a light load the fan speed is dropped by lowering the voltage fed to the fan.
My Asus motherboard lowers CPU fan speed when my reported CPU temperature is lower. Some Antec PSUs drop the voltage to certain outputs which are meant for fans.
If your power supply has worked until now, it most likely isn't just dieing on you. If you haven't noticed the fan loudening before, it is probably because you have newer, more CPU-intensive applications/games, or maybe just you use things with less sound now.
If you are really worried, just take your computer to your local computer shop, and ask them to have a look at it for you. (If your PSU makes a rumbling noise, not a whirring noise, be worried. I am taking my computer to
ARC Computers tomorrow for that reason.)
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