My SO has some mild synesthesia, mostly just lights having a “sound”. Like, our neighbor had some really obnoxiously bright flashing christmas lights that were allegedly quite loud. He “heard” something akin to a tiny truck backing up whenever he glanced out the window at the wrong time. He almost always knows if it’s a real sound or a synesthesia “sound”, though there have been a couple of occasions when he has had to ask me if i hear stuff while watching a show or movie that warranted an epilepsy warning. Sometimes he’ll get a smell when listening music that makes him nostalgic, too, but that’s only happened once or twice.