Vauxhall Insignia knocking from rear

hi there, my problem is really annoying me !! when im driving slow ie 10mph approx. sometimes a little faster, if I turn right or left slow but sharp there a small knock from the rear sounding like its underneath. I have heard it when reversing off the drive onto the road. the car has only done 17,500 miles, the garage have checked it over and say they carnt find anything and its safe???? any ideas guys?? kind regards in advance