the problems people have lol..you are a man so its not a case of a stalker creeping out a lone woman..its also probably not a ghost although I wouldn't rule it out (considering its an old building). You mention there's bachelors living next door. It has to be one of them being a nuisance. Just confront them or put up with it. You could even take it as a joke and knock back as a game..and end up being friends with those guys, your mystery knocker included