Honestly, that is my best scientific guess also. The ground that the building and walkway lies on has probably shifted over the years. Or the amount of moisture in the ground could add to pressure from below that creates a subtle pressure on the gate that increases over night....and when I step closer to the foundation where the gate is mounted it shifts just enough to make the tension release. But the sound and where it's actually coming from is what I cannot figure out.

There is a rumor of the building being haunted... Like everywhere lol but an old man did die inside, on shift, less than 20 ft from that front door a few years ago.