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I raise houses and build foundations for a living, and I can tell you that the pad was not dug deep enough and is now sinking. Your going to have to jack that area up again, pull out the pad, dig to...

Maybe your house was built on a old burial site. Don't worry, its just your rafters shifting and escaping heat and what not.

It is caused by contraction and expansion of the ducts. Maybe you could put something in the center of the vent to keep its shape while either cold or hot (aluminum rod, tight vertical wire... etc)...