Windows 7: Really High CPU Fan RPM

ok... so this has been happening for about the last 2 weeks... lately my CPU fan has been running really loud and fast. I know that when the CPU is hot the fan speed gets higher to compensate.. but my fan is running at 4000-5000RPM on idle when my CPU is at relatively good temperature of 45-47C when a few weeks ago was very quiet and was probably running about half that on idle.'
So i dont know what the problem is... is there any way to maybe fix or would it be best to just go out and buy a new CPU fan.
Heres some computer specs to help:
CPU: AMD Phenom II x 4
Motherboard: GA-MA790XT-UD4P
RAM: Corsair DDR3 4096MB

Are the fins on the heatsink clogged with dust? This effectively reduces their surface area and so in order to remove the same amount of transferred heat the fan needs to work harder (higher RPM).

Check for dust as Dwarf said. And Have you disabled any option similar to CPU enhanced thermal control or CPU fan speed control? Disabling such options will cause the fan to run at maximum speed irrespective of the temperature.

ok... i think the dust my be a bit of the cause because i just checked then and seems like there is a bit of dust on the heatsink however i dont have compressed air to clear it at the moment and as for disable/enable CPU enhanced thermal control or CPU fan speed control where would i do that in the BIOS?

Do you have a bicycle pump? Or even a straw that you can blow down?

Whereas a good blast of air will clear most dust from a heatsink, there are times when this is not sufficient. In these cases, you need to loosen the accrued dust with a small brush (the one that you sometimes get with electric shavers is ideal) prior to blowing it away. At the same time, detach the fan from the heatsink and take the opportunity to clean the blades. Don't forget to reattach the fan afterwards as even with a perfectly clean heatsink a faulty/missing fan can cause the CPU temperature to rise as the heatsink on its own is not able to dissipate the heat generated.

Ya, you'll find them in your bios, probably under tweaking or advanced settings..

well i found a bicycle pump and used it for the cpu fan and heatsink didnt seem to work too well but my computer seems to be running a lot quieter now but i don't think it was because of that.... see if happens tomorrow in the day when its hot

ok... it still is going really loud and high rpm.... i got some compressed air but it didnt really work too well because i couldnt take off fan... it still is going 4000+ RPM when im at lvl 50s in CPU temperature

Iv cleaned the heatsink and fan the best i can with cotton tips and compressed air... but still seems to run very loud and fast on idle anyone got any ideas?