The easiest to check (and the most likely cause) is the CPU fan. Carefully
take off the cover. If the fan is turning slowly, is full of muck or (and
do this very carefully) the noise stops and / or it doesn't recover if you
stop it with a finger, replace it.
Another possible cause is the Power supply unit fan.
Yet another possible cause is a hard disk drive.
Any of these may corpse the computer (with a lot of cost and stress over
lost data) so don't delay.
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