What is the buzzing noise coming from an interior house wall?

Partition walls.

Gypsum. Gypsum is a sulfate mineral that is mostly used for wallboard (sheet rock) and plaster products. It also is used for cement production, agricultural applications, glass making, and other...

Yes; upward, downward, and sideways. Surface texture can make a difference.

I doubt it rust simply by nature, is a slow deterioration and not sudden and accidental, however turn the claim in to be sure.

The simple answer is: When they need it. Todays interior paints don't really fail, they do however get dirty, dinged up and worn away on areas of high use. How often this occurs will depend on the...