from The Frog King, or Iron Henry by the Brothers Grimm
the classic story of the prince that was bewitched and turned into a frog, with a princess helping him return to human form. commonly known now as "The Frog Prince" in English, apparently the original story was named, literally translated, "The Frog King, or Iron Henry." you might ask, why Iron Henry? well, the answer is, the original version ends not with a focus on how great it is for the prince to marry the princess now that he's human again, but with his faithful companion Henry (or Faithful Henry, in some English translations) driving him and the princess off in a carriage back to his kingdom. and the prince and princess hear these cracks happening as they're leaving and they're like woah the carriage is breaking, but no, Henry explains it's the iron bands he bound his heart in to keep it from breaking after the loss of his prince, which are now breaking off because his prince is back and his heart is full again.
this was omitted from later adaptations apparently, because the people adapting it didn't understand what was so important about Faithful Henry and decided they only needed the princess and prince riding off.
according to the Author's Notes for the Grimm's Household Tales, the story is one of many from Germany that were circulating around the time featuring the frog prince and the princess that releases him. notably, one of the stories features the cracking noise in the carriage as that of the broken heart strings of the princess, who is secretly riding behind the carriage disguised as a male soldier, on the way for the prince to marry some other woman.