weird and extremely annoying rattle

Something in the passenger floorboard area (near the firewall it sounds like) starts rattling like crazy when I get on the interstate. It sounds like a pebble or a clod of dirt rattling around in something plastic. It only makes the noise when I'm going at a high speed so I am wondering if it's something in the engine bay or maybe a rock stuck inside my fender. Has anyone else had this problem? It's realllllly freaking annoying and it's not like I can just shove my head under there going 70 mph... even making a friend or something find it is dangerous AF. Help is appreciated.