Your dealer is right. every G.M. vehicle built from about '99 to present is affected by this at some point. the intermediate steering shaft makes a clunk/ pop noise while turning. G.M.'s fix is to lubricate the shaft, which takes care of the noise for a while, but will eventually return. Replacing the shaft lasts a little longer, but will still make noise. You can fix this at home by removing the intermediate shaft ( one end is inside the cab and the other end is real close to # 5 cylinder), be very careful not to rotate the steering wheel with this disconnected as that will destroy the SIR coil for your airbag!!!! Squeeze some grease into the open end of the steering shaft, then plug that end and work the 2 halves up and down until grease comes out the splined area. Then reinstall. When you first remove the steering shaft, try to twist the 2 halves and feel for looseness, there will most likely be none, but if there is any play in it, replace it.