My 2001 Suburban 2500 4X2 has a popping noise in the front suspension. It happens after backing with steering in either right or left full turn like backing in the garage, parking space, etc. The "pop" comes when the steering is returned to straight and the truck starts to move forward. It is quite pronounced and annoying. My mechanic has checked eveything underneath over and over. He found that the drivers side upper ball joint was bad and replaced it. He could find no other problems but the pop is still there. The truck has no driveability problems, just a "pop". Any ideas?