vibration on 3rd gear on deceleration

Hi to everyone i am new member so if i do anything wrong don't kill me ... :)

my audi a6 c5 2004 1.9 tdi AVF multitronic 6 speed 

the problem i do have is on the 3 rd gear when i go let say 2000 rpm and take my feet off is ok till the point when slow to 1600 and the start badly vibrating until goes down enough to change to 2nd gear and vibration is gone i try all the gears same way and all of them are ok no vibration at all just the 3rd is an issue so went to (garage few of them) everyone one says deferent thing lol so maybe here is someone who really know why is that or had similar problem or can direct me what to look for ....