jack up the front and use stands to support both sides. have a helper put it in drive and get it up to 30-40 mph or wherever the noise is audible. Sometimes you can hear the noise if you listen with your ear right next to the spinning front wheel. Try both sides and compare. If you can't tell then use a long screwdriver (a thin one is best) and place the end on the steering knuckle(as you lay under the truck)and listen with your ear on the handle end. Again try both sides and compare for the louder noise.