I have a 2003 mini cooper s, manual transmission. 110,000 miles. I drove it four miles on a gravel road last week. Ever since, I hear a sound like a rock rattling around whenever I go above 50mph. The rattle is under the hood, near me when I am driving, I hear it quite well with the windows up. The car does not lose power, the noise is not affected by RPMs, fluid levels are fine. Am I safe assuming the noise is caused by a rock? Or could the gravel road be a red herring?

 

Just a guess, but you might have loose gravel stuck in your body panels. I would try taking off the skid plate and the front fender liners and inspecting the engine bay for loose gravel.

The gravel stuck in your tires should have come out by now, but check them as well.

 

The one I always hated was getting a rock stuck inside the stock exhaust pipe tips. Seems to take forever for it to vibrate out.

 

not sure if the spring perches are the same or not, but I think you can reach up around the spring and feel a sort of 'cup' that I would assume something could get stuck on and rattle. I remember seeing a whole lot of grit and dirt in mine

 

I've driven on a number of gravel roads recently (stupid GPS thinks it's clever...) and a few times I heard something that sounds like it is bouncing around. It has always worked itself out for me after a little bit of driving. Rocks stuck in the tires can stay there a WHILE if you never get up to any sort of speed.

 

I recently noticed that I have the exact same rattle as the OP. I'm praying this rattles itself away because it's incredibly annoying. The other rattles I can deal with but for some reason this one is really irritating.