Popping? Hmm, difficult since you said you can't isolate or recreate the sound on demand. Try looking at plastic parts to the seat first as these shifting could cause a 'pop' sound. I would start by having someone sit in the seat and rock around while I watched the lower portion of the seat back for movement.

__________________

An unwavering defender of those I see worth protecting.

"promote the general welfare, not provide the general welfare"

We the People of the United States, in Order to form a more perfect Union, establish Justice, insure domestic Tranquility, provide for the common defence, promote the general Welfare, and secure the Blessings of Liberty to ourselves and our Posterity, do ordain and establish this Constitution for the United States of America.