i had replaced the harddisk for an SSD and after turnin it on the fan went to 5100 tpm. very noisy. after reading seeral blogs i tried to remove an sensor of the harddisk temperature bu it was the same. i noticed the wire on the logic board of the harddisk was not so good connected and when i put it together the fan did his work properly and the nois was gone! look at he wire connection on the board connected to the hard disk!