I know what "no da" and "n da" are for. When I hear the question something like "nani wo shite iru no?" from a friend, I wonder: can the question "nani wo shite iru n da?" be used instead? Do they mean exactly the same meaning or is there any difference in connotation?
doko e iku no? --> doko e iku n da?
In other words, can "(na) no?" always be replaced with "(na) n da?" in a question when talking to friends? Will this soound a bit more direct and masculine?