Rattling from Read End

Hey,

I've got a 2007 G6, 4cyl. and have noticed a rattle coming from the rear end somewhere. Not present when in park, but when I idle forwards or backwards, there's a noticable rattle. I've tried hitting the gas and brake pedal at the same time, to see if it's a function of the idle speed, but no dice. I don't see anything obviously loose back there - has anybody seen this before? Any ideas?

Thanks!!