I've just found what was causing the rattle that was driving me mad in city traffic. It's the front grille, and all the screws are done up properly.
For some reason, the bottom side of the grille is somewhat loose. It's got 4 plastic hooks that go into catches in big front plastic assembly, but they're not really tight even when top screws are tight. Will try some electric tape on them to make them tighter, but that's really odd.
Anyone else with grille rattling? Rattles more when in Drive + all headlights, A/C, and fan on full (probably engine vibrates more because of more load).