What's that knocking noise?

Two days ago I purchased a used 2006 Acura MDX w/33,000 miles. It drives very smoothly on the highway, but when I’m driving around off the highway, there is a knocking noise at the drivers side. I hear it when I’m stopping at a light, turning onto a new street, or driving in a parking lot. It’s not consistent, and just has a knock every now and again, almost like an inconsistent sound of riding up a roller coaster.