2003 Highlander 4 cylinder
Seems to only happen in the fall or early winter here in the San Francisco Bay area where I live
in the early morning when it's still cold - and it doesn't happen every morning :
When I go between 25 - 35 miles per hour only
there is a loud rattling on the right side(towards the rear).
Even if I take my foot off the gas and shift to neutral the rattling continues if the car
is still in the 25 to 35 mph range.

I tightened loose screws on the overhead luggage rack and haven't had the problem since then.