2004 Buick Rendezvous clunking noise from front of vehicle

Hello,

Sometimes when backing out of my garage and turning the steering wheel I hear a slight but definite clunk from the front. I sometimes actually feel it come through the steering wheel. It will also happens when I turn into a driveway off a main road, when braking or not braking. I only hear it at very slow speed sharp turns either left or right. I took it to my local Buick dealer and they could not duplicate it, but it was raining that day and after the underside of the vehicle got wet, I didn't hear it either. My extended warranty runs out on June 19th and I'm affraid I'm going to have a major repair bill when watever is clunking finally breaks.
Thanks in advance for any advice you may have.
Bob