What causes metal to creak?

I am a welding student so I do indeed know alot about steel's/welding metals. In steel there is a cell like bond. Such as in anything else. Once heat or strees is applied you force the steel to change cell shape causeing the creak you hear, its mostly stress of the steel. This can be prevented by following pre-heating guidelines or limiting the stress on the steel. And do remember everything has a limit...so dont push to much! hope i helped