Is it a loud clicking or does it seem to happen when the HDD is active? Some HDDs do make noise when under use, Seagates normally don't though. I have 2 of the 2TB 1TB platter HDDs and they make a slight clicking when being used but I doubt it is that.
As said, it could be the PSU fan.
One thing to check, when you installed the CPU fan did you take the cable and bunch it up off to the side or did you leave it wrapped around the cooler?
I have seen where that will get a bit loose and hit the CPU fan.