What do gut feelings feel like?

I have a gen friend sacral authority who is into human design and in a toxic relationship. When I talk to them, their S&A gut feeling is to leave the relationship. Then later their S&A always changes to stay. They don't really know what a gut feeling is supposed to feel like, and they have asked me but I have no idea as a Projector. So... what does it feel like? Any metaphors that might help? I've heard about the sound thing - the humming idea... how does that work? i've also heard that a question is best asked by someone else in a yes no format. Any help would be appreciated, they are in such a pickle and I just want them to make the right decision, whatever that is, because it's a big one with big ramifications!