In my car (focus) it turned out to be the drop links on the front suspension which were worn and allowing movement in the anti-roll bar. One end of the link is attached to the strut and the other to the end of the anti roll bar (one each side!). I replaced them myself and it solved the problem. Like you, I had mechanics check the mountings, front and rear, to no avail. The drop links are a common problem on the Focus - so I was told by the motor parts shop which keep a good stock of them. To replace them you need a few different spanners and you need two of the same size - one to hold the nut to the rear while you loosen the nut to the front. Annoying job but easy enough to do with patience!