vibration from rear axel when acceleration
 

I have a kind of vibration sound, like some gear slipping (klak klak klak) not too lound coming from the back (I think) when hard acceleration (0-120) after that it seems to disappear.....does not happen at high speed when I accelerate hard. does not happen at low acceleration (normal), any idea?

I have a 2010 S4, with 110,000km on it, stock no modif done