What causes a rattling sound underneath the front left side of a car while driving?

The noise that you are hearing is telling you to stop. The longer you wait, the more expensive it we be to repair. Right now you might get away with just a bearingreplacement. This could turn into a complete hub and spindle and bearing replacement. The bearing could lock up entirely and snap off. This is not something that you put off. At freeway speed, you could cause an accident if it fails at high speed, you could loose control. Do the right thing and park it until you can afford the repair.