According to the poster on Reddit, “The aluminium backplate is a lot thinner than on the iPad 4 which I also have. You can almost feel the battery through the plate when you hold the device. Both iPads have the same feeling and are making creaking noises when you hold them.”
It is unclear if this is an issue that is affecting all iPad Air 5 units, or if it is only a few that are experiencing this issue. It is possible that Apple could have made some changes to the internal structure that would cause the creaking, or alternatively, the historical high prices of aluminum could have forced Apple to use less material to keep costs down, which could explain why it might flex more compared to if Apple were to use a thicker slab.
Keep in mind that these are all anecdotal reports so it is unclear if Apple did indeed compromise on the material, so hopefully a full teardown of the tablet or a statement from Apple will help clear the air. Apple did previously face some controversy with their iPad Pro several years ago when some users discovered that their tablets were shipped bent, although the company did state back then that they did not consider this to be a defect.