If nothing so far has led you to finding the rattle, then you probably need to work out what conditions cause the noise, and what sort of noise it is.
You have called it a rattle; is it a nut in a tin-can rattle, or a drumming pencil against the bodywork, or a heavier weight thudding against solid end-stops?
And do you get it when drive smooth and flat, bouncing over undulations, jumping over traffic bumps, cornering, etc?
Can you cause it by accelerating or braking?
And if you open the window and listen to the reflected sound from a wall, do you hear a different sort of noise? (This would confirm the noise is outside the car)
And don't assume that just because the bolt won't turn to tighten the spare tyre mount, that the tyre mount is tight. You shouldn't need it very tight; remember, after a few months, it will rust a bit, and then you will need to get it undone again...