As people said on posts above me. It seems like it's happening since awhile ago on some units. I got it on mine and doing what I did fixed it. It's still fixed till now. Hopefully it won't come backI have the same problem with my 16.2". It's happening almost exclusively on my lap whenever I adjust the machine or my hands. It is much more noticeable when the lid is open then closed, but its happens at both times. It seems to be mostly emanating from the right side of the bottom panel (belly pan if you will) and I can constantly re-create it by holding the laptop in open in both hands and positioning my right hand with my thumb on the right speaker and squeezing the bottom panel. It is a creak/squeak like sound. I have had this on other MacBooks but I think this the worst I have heard by far. I plan to call Apple and seek a remedy, I don't think it's right that a machine this new and this expensive creaks at random and to the point it's audible to others sitting near me.
..... Unscrewed every screw from the back. Then putting it back together tightly seems to do the job. Since then no creaks at all. Crossing my fingers it won't do it anymore......