Also experiencing the same rattle. Mossy Toyota in SD reports it's how it is designed. Dealership opened a case with Toyota Engineering to confirm. It's extremely loud on my vehicle and I can hear it with windows closed when accelerating (faintly but def there). Sound begins at 13/14mph. Persists through 3rd or 4th gear. I'm trying to get more precise with parameters of the noise. I really hate it!