Hi this sound you hearing could be a symptom of a HDD failing soon, so back up what you have or important stuff external first!, i could crash anytime, another symptom to look out for is computer is lagging and slowing down, then followed by clicking sound until one day it decide not to run anymore, so play it safe and back up first, then try and do a disk check to check for bad sectors, just make sure you observe the sound carefully and often, cheers!