Why does rear suspension system keep adjusting itself? on 2007 Buick Rainier

Vehicle is not sitting low like in some other cases mentioned. Also notice a clunking sound sometimes when going from one surface level to another, i.e. backing out of garage which has a lip onto the driveway. Sometimes sound comes from left front of vehicle, other times from rear right of vehicle.

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1 answer 2 comments
poss air leak at bags or lines,very common and costly issue
When you say "costly", what's a ballpark repair cost?
if it needs everthing poss 1500.00