As a professional hardware support person (puts on CSR hat) I would....
Determine if only 1 of your processor fans are 'revving up and down' or if they both are. (it may just be a faulty fan or the settings for 1 of the fans is incorrect) At this point I would also determine if it wasn't the fan on the video card that was actually making all the noise. And make sure that the fan on the CPU's was rated correctly (ie. someone didn't cheap out on the cooling fan)
Then I would look is in the BIOS settings for CPU temprature settings (or in the software that may control this, if any) they may be set too low. You need to get the proper specifications from your CPU manufacturer and set your sensitivity accordingly.
If those settings were correct I would check the airflow in the chassis - make sure you dont have a bundle of cables blocking an important airflow point.
Then check that all the other fans in the chassis are functioning correctly - one of them may not be working (don't forget the power supply fan)
Then make sure your Chassis is not in a confined space - it needs lots of space to breathe.
Then I would ask the user if thay went and 'overclocked' their CPU - If you answer yes to this I would say.... well, I wouldn't say anything, probably just laugh and walk away....
But if you did 'over clock' you need to 'up' the power in your fan because it will definatly not be rated for the extra heat you will be pushing out.
Hope that helps
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