ways of reducing noise of CPU

1. Use sleeve fans rather than bearing fans when possible
2. Check dba ratings on all fans you use – from the CPU fan to the case/chassis fan to the PSU.
3. Be aware that many components that come with fans are also available in no-fan versions - including motherboards with just a heat-sink and no fan on the Northbridge - and power supplies that are based more on music system power supplies and don’t need active cooling.
4. When using fans use larger fans with a lower rpm. A 120 mm chassis (case) fan running at a low rpm will generate the same cfm (cubic feet per minute) of airflow as an 80 mm fan running at a higher rpm, but will generally make less noise.
5. Avoid using PCI slot 1. Keep some distance between the graphics card fan and other PCI cards so air from the graphics card fan will not be obstructed.
6. Some hard disks are sold as “Quiet” drives, they tend to not cost any more than standard hard disks. Shop around for quiet drives.
7. 5400 rpm hard disks may not be quieter than the low noise 7200 or 10000 rpm disks. Higher rpm generally mean more whine BUT many of the higher rpm “performance” hard disks use fluid dynamic bearings and other clever technologies to run very quietly indeed.
8. Route your cables carefully. When they block airflow they add to the noise.
9. Choose your case carefully. Buying a quality case will allow you to add other sound control features later.
10. Use the right wattage of PSU. If your PC requires a 350 Watt PSU it tends to be neither quieter nor environmentally friendly to use a 550 watt one.
11. If you have grills on the case they may look pretty but if they have a chassis fan behind them they will disrupt the air coming out of the fan - and that makes a noise.
12. Use filters over air vents for the air intake fans. Dust getting into the PC will make the fans noisier over time. (Washable filters are obviously preferable to the throwaway ones)
13. Identify all the moving parts and make sure they are secured well and are not vibrating. This goes for everything from the fan screwed onto the CPU heat sink to the optical drives, hard disk, chassis fans and even the PSU. Use tie wraps and other securing mechanisms if necessary. They can even be used in addition to the normal retaining screws on devices like optical drives
14. Identify other parts that could move or vibrate. Securing the hard disk firmly is not sufficient if the hard disk carriage/cage moves about or rattles. Secure the cage with tie wraps.
15. Be always conscious that heat is a killer and if you compromise on heat dissipation then parts could burn out and the overall lifespan of your PC will be lowered.