[dianenancy]

Makes a popping noise when you turn the wheel sharply to the left when going slow.

[TxGreaseMonkey]

Check the constant velocity joints. When you turn the wheel sharply to the left, it "loads" the joints. If a joint is bad, it may make a popping noise. See if a boot has split and grease has been thrown all over the place.

[jershu3]

An Excursion would have Universal joints in the front axle. The easist way to check them is to jack the front up so that the wheels free spin. First turn the wheels all the way to one side, and spin the wheels by hand, and see if either bind while turning, then carefully get under the vehicle with a small prybar, and pry up slightly on all four sides of the u-joint. There should be little to no slack in the joint. Watch for excess movement in the ball joints while doing it. There should be some, but not much. While it's up push left and right on both of the front wheels, again there may be a little slack but no one joint should have a lot. It takes less than a dimes thickness to make a lot of noise.