Yeah, that's understandable. I kinda wonder too but I think only time will tell. If it bugs you, call apple and swap it out. My wife's 5s makes no noticeable noise when touching the touch sensor. The home button has a crunchy double-click feel sometimes. Doesn't bother her and it still works going on a year.
I've played the whole "exchange for some minor non-issue" game before and it's unnecessary work in my opinion. I sent a 4s back for a camera lens that rattled like marbles when you sat the phone down. No noticeable impact on the camera, but it SOUNDED bad to me. Idiot. The next one was much more quiet and I was happy. Then the phone was given to my wife and promptly stolen.
Now, I carry my phones without cases and have applecare plus. I had 3 iPhone 5 models for various issues. I can't be sweating the small stuff. There's a good chance this will not be the same physical iPhone 6 that I have in my possession in a year.... things happen.
Do post back and let us know what you decide to do! I'd like to know how common this trait is.