I'd recommend you check both HDDs on another computer, @Eijon, and see if they will continue to click there as well. Clicking noises, as I mentioned, indicate hardware failure on the HDD itself. However, it is also possible that the PSU could be faulty, too. You should
definitely test the power supply unit as well and see if there's anything you should be concerned about there. However, I'd still advise you to power down your system and reseat all the connections inside. Make sure everything is properly plugged & mounted.
If you don't feel comfortable doing this by yourself, you should find a PC repair service nearby and let the guys there perform full diagnostics on your system. They will easily find the source of the problem.
Let me know how the HDDs behave on another PC!
SuperSoph_WD