The clip is pretty obvious; usually visible with the wheel on. #7 in the diagram below:
If it's missing, the noise is usually a clattering when you hit a bump. The clip is designed to keep tension on the pad so that it doesn't rattle. In looking over the symptoms you've described it doesn't sound like that's the problem though. It might be a loose caliper or something broken. Best bet is to pull the wheels one end at a time and check things over carefully, or take it to a shop and have it inspected.