If a fan has got something in its bearing or the shroud has warped and is making contact it'll be very noisy. Though the HDD is another possibility as Ethermaster mentions.It sounds like I am tapping my fingernail on metal, could a fan be that loud??
Take the sata power power cable out of the hdd and start the system, if the noise is still there its not the HDD, if plugging the hdd back in and the noise returns its new disk time and to start making backups of everything.