makes this noise every time it’s on

Every time I turn my IBuyPower PC on, it’s been making this loudish clicking noise in the back of it. It’s been like this for months and is driving me crazy but I can’t figure out what the problem is… I took the fan out and cleaned it and the filter behind it… no difference.. we even messed with the fan itself a bit to see if it’s the actual fan or something behind it? Help please lol