A "Hey Arnold!' episode question.?

I can only remember bits and pieces of it, and it might not be just one episode. I want to know what season it is in, or at least the episode name. I think they were talking about the headless horseman, and they thought that there was something in the basement, because they're hearing strange noises. They go down and it's one of the people in the apartment. Sorry it's so vague, it's probably been more than a decade. Thanks.