Chassis Pop Noise

[Lyndongolden]

My 2008 Sierra (regular cab) has a "pop" under the rear of the cab when accelerating from a stop and turning at the same time.

 

This is one brief pop noise...it sounds like someone is lightly tapping the bottom of the cab / frame with a hammer. It comes and goes.

 

The dealer adjusted and shimmed the cab mounts several times but the noise always returns in about a week. I lubed all the rear leaf springs, bed mount points, cross-member contact points, and cab mounts. Last week, the dealer replaced the leaf springs and yet the "cab" noise is still there.

 

HELP!