I've had the same noise coming from my front end a few years ago. It ended up being the lower control arm (the expensive one, of course) on the driver's side. It also would occasionally make sounds you would expect a bed to make when being used for 'wrestling'... lol. When you hear/feel the noise, does the car feel like it's shifting? The best way I could explain it would be like going over a bump for a split second on one side and straightening itself out.