When I make music I might get an idea of a rhythm or potential "up/down" pattern to chords/notes etc, but I'm bit sure I necessarily hear music for song ideas until I sit down and bash shit out for a few hours. You mention writing as coming with words and music together, do you just hear things in your head or does it come from a lot of improv and chasing a certain idea?
i can do it both ways. but either way, i've found that the idea usually decides when it wants to come to you. all you can do is try to make your mind seem like a friendly and inviting place, so then hopefully the idea will want to go there