I have 2006 ES330 (16,000 miles). I hear a sound from the back when I do hard stop and also hard acceleration (only this combination). The sound is almost like a basketball is hitting the trunk wall (although it is not loud however you can notice this sound during night time or quiet street). When I do normal stop and normal acceleration (or hard stop and normal acceleration)then there is no sound. It only happens hard stop and hard acceleration. According to the dealer it is due to brake caliper moving when I do hard stop and hard acceleration. It happens when I back up the car as well (well only once or twice). It does not happen all the time and again only hard stop and hard acceleration. What do you think? Is this normal?

 

Stuff moving around in the back? Tool area/spare tire..

 

Yes, first I thought about that possibility. Although my trunk is empty, I still have spare tire and tool box still in the trunk. The sound is definitely comming from the back.