My iphone had the issue since I bought 3 weeks ago. Let me tell you kind of my story, this is what I have seen in my iphone, it might be or it might not be I have no scientific proof. I started listening to the sound in the bottom right corner, sometimes it was very obvious, sometimes you couldn´t listen nor reproduce it. I went to the genius bar, which by the way they have become really arrogant, I had apple products since long long ago and customer service is not as expect these days. I went two times, the first one I couldn't manage to reproduce the clicking noise, so they couldn't help me. The second time however I could, he tightened the screws in the bottom after arguing with him, since he said that it was very normal. The problem was solved.
BUT next day I was out in the sun and the iphone started doing it again, not that much as before but it did. So I started testing the iphone and I discovered that at least mine, if am in cold or ambient temperature my iphone is ok, no clicking sound, if it gets hot it starts making that clicking sound. I have reproduced this a lot of times, my iphone 5C gets hot when charging, and you can easily hear the clicking sound. If I disconnect the iphone and leave it there for a while, it is gone. So maybe the thermal expansion of the plastic apple is using for Iphones 5C is not up to for the task. Again it is purely based on observation, no scientific proof no nothing, just my experience.
I´ve had no problems however with my iphone, other that that.