hahaha that thing about the sc coupler making a noise is hillarious considering the guy that had mine before me said he replaced the whole sc because it was making a noise and would be harder to sell that way. he said it wasnt broke, he just replaced it to try to get rid of it. i bought it from a dealer, but not a very smart one obviously.
something else that will cause a rattle at idle is the balancer. mine started to rattle at idle and i figured it was the coupler, it took me a week to to get parts to replace it and just to make sure before i did it, i checked everything again, here the rubber on the balancer was broken and was causing it to rattle.. GM redesigned it with a narrower rubber in it now.