What you are hearing is the difference between a steam locomotives with the valves slightly "out of square" (1225) and a locomotive where the valves are set perfectly square (765.) In the 1225 video, you can also hear rhythmic "squeaks" and "whistles" from the engine, which is steam blowing by the valve rings.
The 1225's valves are worn and need a little work, that's all. At one time, the 765 sounded like that, and even worse!
