My car has an occasional rattle that sounds like it's coming from the engine bay but I haven't stuck my head out the window to see if it's louder outside yet.
I've only noticed it since it's been colder out and it only happens when I first start driving the car, not when it's stationary. I've lost a couple of washers in the engine bay so I figured it was those but I can't remember if I noticed the rattle before or after I did that...