Not an easy one to find, but if its passed its MOT then its not anything serious. One thing that it may be is the exhaust system, in that when you are turning then the cars weight transfers and it might be that the exhaust pipe is knocking against the chassis somewhere underneath. If you can get to look under the vehicle (ie pit or ramp) look to see if there is a mark on the back end of your exhaust, especially where it comes close to the rear suspension area.