2002 Grand Caravan Front End Pop

I have been hearing a noise that seems to be from the left front end. It sounds like a stone hitting the car--or like if you hit the fender with your knuckle. It is intermittent but when it does happen it is almost always within the first 50 feet or so of travel. It happens only once per trip and is usually associated with turning the steering wheel, or going over a small bump in the street. I've had it to the dealer and the mechanic can't find anything wrong. I'd like to get it repaired soon as my extended warranty runs out in a few days when I hit