A) Processor fan speeds up to cool the Processor.
Gaming, a video, and uploading requires more Processor 'power'.
A Processor has a Thermal Limit. A temperature it can reach, and no more.
Going past this thermal limit would burn the Processor up.
The Processor fan speeds up, to keep the Processor below this thermal limit.
(If the Processor temperature exceeds the thermal limit, the Processor is turned off. BIOS turns it off)
The Processor fan is usually one of two methods used.
1) A fan sitting on top of the Heatsink, which sits on top of the Processor's case,
Typical Heatsink/Fan combo,http://www.tigerdirect.com/applications/SearchTools/item-details.asp?EdpNo=3451342&CatId=493
2) A computer case fan in the front, that pushes air through a plastic shroud, which envelopes the Heatsink, sitting on top of the Processor.
Perhaps the fan's bearings are going out.
Bad fan bearings will let the fan shaft tilt to one side, and the fan's center hub will scrape on the fan motor.
This can cause a whining sound.
(Goes for examples 1 and 2 above)
Computer unplugged from power, Anti-Static Precautions observed, see if you can gently rock the fan's blades up, and down.
NO perceptible movement should be observed, nor felt.
If so this fan needs to be replaced.
If the inside of the computer is dirty, the added 'Gunk', (Dust, dirt, hair, you name it), will cause the fan blades to be unbalanced.
This will make the fan tilt to one side also.
Computer unplugged from power, Anti-Static Precautions observed, computer case open, use a can of compressed air for computers, and clean the inside of the computer out.
May require two cans.
May also require that you use Q-tips to disturb the outer layer of the 'Gunk', so that the can of air can remove it.
(Would also like to add:
The can of compressed air for computers comes with a plastic 'straw'.
The straw is attached to the nozzle of the can of air, when using it.
A Power Supply should have the inside of it's case cleaned also.
The cooling components for a Power Supply, are it's fan, and internal Heatsink's used inside.http://en.wikipedia.org/wiki/Switched-mode_power_supply
[ Typical construction of a Heatsink is a plate of metal, with tall, thin fins protruding from it.
The plate of metal absorbs heat from whatever object it is placed against, and the tall, thin fins absorb heat from the plate of metal.
The tall, thin fins then radiate the heat away.
If a fan is used in conjunction with a Heatsink, (Such as a Power Supply, or Processor), the air flow from the fan helps to carry heat away from the fins ]
When the fan blades, center hub, and surrounding shroud are coated with Gunk, and the Heatsink's inside a Power Supply, their cooling capacity drops TREMENDOUSLY.
This causes the Power Supply to overheat.
Heat = Wasted Energy.
The Power Supply strains to keep up with the call for power, and eventually cannot.
Components inside the Power Supply fail, and the Power Supply fails.
(As in Kaput)
The Power Supply's case has ventilation holes, on the case side inside the computer.
The plastic straw of the compressed air for computers, is inserted into the top ventilation holes, and air is shot in. (One ventilation hole at a time)
The straw is aimed in a slightly downward fashion.
Then the straw is used in the lower ventilation holes, and then on the bottom ventilation holes.
HOLD onto the plastic straw with one hand, and the compressed air for computers with the other hand.
The straw can shoot off of the can of air's nozzle, and wind up inside the Power Supply's case.
This = NO
No go to the back of the Power Supply, and use Q-tips to get into the crevice where the fan blades meet the center hub.
Also on the edges of the blades, and the surrounding shroud around the blades.
You just need to disturb the outer layer of Gunk, so the air can remove it.
Shoot air past the fan into the Power Supply case also.
You won't get the inside of the Power Supply perfectly clean, but you will extend the life of the Power Supply greatly.
The inside of the computer, and the inside of the Power Supply, should be cleaned on a regular basis as needed.
Different scenarios require more frequent cleaning, or less frequent cleaning.
Only you will know, when you check inside your computer on a regular basis.
You can be the best housekeeper in the world, but even in pristine homes, and businesses, the inside of a computer, (And Power Supply), gets dirty.
(Carpet deodorizer can clog a computer too. It's a fine dust)
If the inside of your computer is kept pristine clean, and you have No fan bearing problems, then your Harddrive is on it's way out.
This explains the workings inside a Harddrive,http://computer.howstuffworks.com/hard-disk.htm
(The Spindle motor's bearings are going bad )