I have been hearing a rattling sound on my Van and it was coming from underneath, on the front passenger side. I finally got a chance to check it out yesterday and as far as I can tell, it's coming from a plastic splash guard, in the wheel well.
I checked everything else and nothing was loose, so I’m almost certain that’s what it was. Last summer we had to remove the splash guard, in order to change the Drive Belt, tensioner pulley. It’s held in by plastic plugs and once you pull them out, they’re never as tight, when you put them back in. So I will probably end up replacing the plugs, in the spring.