The Interval Signal on Shortwave, that would give you a clue, that the broadcaster, BBC or whomever was going to be on the air with their main programing.
Radio Havana’s was great. I do think it could be a few notes from the famous song, ‘Guantanamera’.
https://www.youtube.com/watch?v=UeJOYSJ5mzQ
Also caught the Beeb’s, good also.