Fan Speeding Up and Down Radomly Throughout Windows

[acurasd]

What would be some things that would cause something like this? That way I can go through the computer to check them out. I was thinking it may be over heat on the processor but usually by default the computer would shut itself off it it gets way too hot.

 

[ignatzatsonic]

What fan? PSU? CPU? Case?

Are you judging by ear or are you reading the RPM from within the BIOS or software?

Is the fan in question a PWM fan?

 

[acurasd]

Welll give me a min to open up the case.. sounds like a case fan... it was pretty loud and coming from the side..
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update
Let me get back to you on this question to see if it still does it... I got this computer from someone and I just opened up the case... there is about an inch of dust and dirt inside...

It is the CPU fan that is going up and down. I got to take a turbine blower to this thing.

I took the cover off and it was like when you are in the attic and you blow on something really old; I got hit in the face with that much dirt lol
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JUST A QUESTION:

Once I get this thing cleaned and the CPU fan continues to go up and down in speeds like its revving an engine; what would be some likely causes of this?