I occasionally experience very strange feelings that I have sometimes labeled nostalgia, but it’s some kind of unknowable nostalgia. The way you describe it seems like the same thing I feel sometimes, especially how it’s familiar, old, almost frightening, but very good. I also wondered if anyone else ever feels that. I never thought I’d hear someone else describe anything like it. I don’t have any insight about that kind of feeling, but I’m glad you posted because it’s a comfort just to know someone else has experienced it too.