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I installed a cooling unit for a client. A couple of months later he called me out on a breakdown.
When I got there (China as it happens!) he had had an airtight enclosure built around it because 'it made too much noise'
There wasn't much left of it.
Open the case in standby mode. If the CPU fan has stopped, then it must be the power-supply. If it is still running and is easily accessible, stop the CPU fan for a moment with your finger (on the hub not the blades).
If it is the CPU fan that needs to be replaced, it would probably be best to get one fitted in a shop. It sounds like your current one is not installed correctly - probably drawing power directly from the power-supply instead of the motherboard. This would explain why it is always on.
Otherwise, you can get fans with either built-in or remote temperature sensors. I like the Thermaltake Smart Case Fan. They are very versatle. They come with everything you need to EITHER have them controlled by temperature, or else with a potentiometer. And (though they don't tell you this) you can even diddle it to use BOTH. (Controlled by temperature, and you use the pot to control the maximum speed.)
Keep in mind that bigger = quieter. A bigger fan can spin at a lower speed to move the same amount of air.
Whatever you do, make sure that the fan has ball bearings, not sleeve. Last thing you need is a fan seizing-up on a modern, power-hungry CPU.