All computers have a cooling fan over their internal processors to keep vital hardware from overheating. Some custom cases take keeping things cool to a whole new level by using an extractor fan. An extractor fan is built into the back of the case, and helps to suck out any warm air building up around your hard-working processor. In almost all cases, extractor fan problems are caused by something blocking the normal movement of the fan blades. A noisy fan is usually just in need of a good cleaning.
Turn off and unplug your computer. Disconnect all associated hardware and cables (monitor, mouse, keyboard). Remove the side panel of the computer case. This may require you to use a key or to remove screws.
Unscrew and remove the plastic air duct that leads from the compressor fan to the extractor fan. Not all cases will have this feature. If your computer does not include an air duct, skip this step.
Check the power cords connected to the extractor fan for looseness, and plug them firmly back into their power leads.
Examine the other cables near the fan for looseness. It's possible that a loose cable or wire associated with another component is hanging in such a way that it blocks the fan blades. Use zip ties or electrical tape to tie any cables out of the way.
Use the compressed air canister to clean the fan. Blow away any dust and debris that is stuck in the blades. Some particularly dirty cases may have large dust bunnies, which can be carefully removed by hand. Use the opportunity of having your case open to clean all of the internal components.
Replace the case panel and air duct (if applicable).
Do all you can to prevent static discharge when opening up your computer case. Wear an antistatic wrist strap.
Never perform computer repairs on carpet.