I just posted the same a topic yesterday on this problem I have, seems like the same problem I have. Only heard this sound around beginning of 4th year of car. Can hear one single "thud" sound when hitting around 10 km/h, then a continous "thud thud thud thud thud" (5X) around 30km/h. Only when the engine is restarted, subsequently during the ride do not hear the sound anymore.
Is this normal?