It’s the sub-floor! Up after time the sub-floor of your wood flooring can come unsecured from the beams they were originally attached to. This is an easy fix if you have the sub-Floor exposed. All you need to do is get some long screws and secure the plywood back to the beams. You have to make sure that you are hitting the beams below to ensure the sub-floor will no longer squeak. Let me know what questions you have!
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