"Tara-ta-tang-tang" has no meaning. It's just a sound one may create when he/she hums a tune. I put it here because it rhymes with the word "tang-tang" (to remove or detach) which is also found in the song. This is filled with a wishful thought that he (the lover) and the "object" of his love will not be "detached" from each other. He also expresses his hopes that the love they have found and shared together will not go away from them.