sound is coming from behind that, closer to the edge of the top of the rear wheel, but a bit higher around the passenger back side of the convertible top under the base
I have a similar rattle but only with the top up. Can't find it and drives me nuts.:ah:
Check the oil filler tube, dipstick tube and anything else coming from the engine up to the access area in the rear trunk. These things can all break and rattle like crazy.