Hmm those things can be hard to track down. I too have had a rattling sound come and go on my car and I can't quite determine where it's coming from. My first thoughts were maybe the cats on my midpipe breaking apart.
It's a stieg just not a stieg lol. I can send you pics. Just a copy cat blower. Shoot me a text if you like. 832-622-1191. Don't judge it until you see pics.