NOTA,
Best thing to do is firstly, remove the PC from whatever corner it is hiding in and then remove the lid.
Pop it up on a convenient chair or table and then et it run with the lid off. Hopefully, the clicking will be more distinct and you can peer in and identify the location.
As I am typing this, my WD external hard drive has just clicked as the heads parked on its way into hibernation... sounds awful, but doesn't do any harm.
Regards,
Shuttlebus
P.S. BTW, have you got power saving switched on, with things such as "Turn off hard drive after" activated... thid could explain the clicking.....