I just brought in my 5c into the Apple store because of the same issue. I couldn't recreate the sound when I was at the store (of course). I told them that there's a creaking/clicking sound when I use the touch screen. And I told him I didn't know I bought a Blackberry Storm (the touch screen where you actually have to physically click). The guy brought it to the backroom and started playing with it. He confirmed that there was a hollow popping noise on the left side when using the device and brought out a replacement for me. He told me to play with the replacement for about 5 minutes and had me leave with a replacement device. He told me that there should be no noises like that when using the phone.
I would say try again. It's not normal.
I would say try again. It's not normal.