I had this once a while back. There is a box built into the tub behind the inner fender. Inside it is some type of vacuum or emission box (I didn't look it up to see exactly what it was, maybe someone else can help out) that had come loose and would knock when turning or over bumps. I removed the battery and put a new nut and bolt on it and tightened it back up. Hope this helps. It was an easy fix.