Shut it down, unplug it form the mains and open the side panel. Get in there with a hoover
(thin nozzle attachment) and clear out all the dust especially on the radiator. Also
hoover the outside front and rear panels to clear the dust that collects in all the little
holes.
The reason your fans are going haywire is because your Mac is bunged up
and fighting to cool down, mine was the same a few days ago until I did the same with a
hoover and now it's quieter than a mouse!
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