This is not the sound of keys in a bag. In older British English, rattling in front of a story — very often rattling good — means the story is lively and exciting: it moves along, and you want to keep reading. A rattling good yarn is an old, friendly way to say a really enjoyable tale. The word feels informal and a little old-fashioned; a young speaker, especially outside Britain, would probably just say a great story. Though actually, rattling here is doing the job of very: rattling good is very good, with extra energy. It almost always sits before the noun, and it has a warm, old-book flavour, not a technical one.