I've seen a few people mention this before but wasn't able to find out any more info.

Every time I go over a speed bump faster then 2-3MPH There is this terrible thump / rattle that sounds like a rusted out 1993 Buick Riviera with blown out shocks.

Was anyone able to determine the cause or how to fix it?

 

When parked, engage the foot brake for a few days. Should go away. Good luck.

 

I have the same issue as you. I sometimes feel embarrassed because people do stare.

 

take it to Lexus. should be covered under warranty.