What may be the cause of a loud noise when riding?
My suspicion turns to the spokes. You've said they're a little loose, and one looser than the rest could easily make a noise only when there's weight on the bike, but with no need to pedal. A slight tighten of all the spokes on that wheel might sort it out, while keeping the wheel true. You may also be able to feel/hear a loose spoke by plucking, and confirm by putting something like tissue paper between that spoke and the ones it touches. An unusally tight spoke may also be the cause.