First of all, sorry for the long post.
My last 2-3 iPhones have all had some manufacturing faults. I have been wondering if there is some difference in quality between factories where these are manufactured? I know my experience is scientifically meaningless but for example my previous iPhone 7 had minor scratches on the screen and slight imperfections in anodized back right out of the box. Based on the serial number this unit was from Zhengzhou - Foxconn factory (serial number starting with F1). However we bought phones with my wife at the same time and she selected different colour and it was from factory Chengdu - Foxconn (serial number starting with DN). That one was flawless in every way. It even had much better screen, it was completely even and I could not find any marks or scratches on it.
Just this week we upgraded our phones to model X. Again this one was based from the serial number from Zhengzhou - Foxconn factory (serial number starting with FK) and AGAIN mine had small scratches on the screen (at the same location as my previous iPhone) and this time glass back had few scratches on the curved part near stainless steel rim (they were actually so deep that I could feel them with my finger) but they mostly show up when looking them under good lightning.
Again my wife selected different colour and it was from factory Chengdu - Foxconn (serial number starting with DN). And AGAIN this one was flawless in terms of any markings on the case. Also it had PERFECT screen, no unevenness what so ever, even with darker near black greys. Display on mine is pretty nice (but just about tolerable for me as I'm very critical viewer when it comes to displays) but it is darker and murkier looking on the bottom and especially with near black greys are terrible in uniformity (bottom part of the screen dark and upper much lighter), it also flickers when it is adjusting brightness automatically when brightness control is going all the way down. My wife's flickers much less when adjusting.
However both of the phones are makings more or less "sounds" when gently pressing or twisting them from the sides and wife's phone also making that sound when tapping glass back at certain locations. I'm not sure if they are supposed to be like that? I was not really twisting the phone or anything when I first noticed that. Mine would sometimes make cracking sound when picking it up and then I noticed that it sometimes make that sound when doing mentioned procedures but usually it does not do it twice in a row at the same location but once you have pressed some other locations. My wife's phone however is squeking like sneaker shoes (not very loud but it makes it all the time when very slightly twisting it). By twisting I don't mean using force to actually twist or bend the phone, just holding it that way and using only slight twisting force.
I know I could have returned mine but phones are not bought directly from Apple shop but from authorized reseller, so only choice would have been claiming manufacturing fault and having them swapped but as far as I know they will not be swapped for brand new retail box but instead you get white box refurbished phone. I know many peoples say they are like new but as far as I know they are actually faulty phones that have been fixed and replaced with new casing to make them look new. Well for me that is still a used phone and I'm not very keen to idea having just bought spanking new expensive top of the line phone and having it swapped to "used" phone. So I guess again I'm going to just try to live with what I have.
It definitely looks to me that from now on I will be very cautious when buying a new phone or anything else (as I had many problems with my iPad too) and definitely I'm trying to avoid phones with serial starting with F.
Just out of curiosity since thread starter had serial numbers of the phones he tested maybe he could list the first letter or two from the serial numbers of those? It would be interesting to see what factory they were from.
afterall.... 
