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.

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All The Girls I've Loved Before:
2006 Mustang GT | 2004 545 | 2004 645 | 2004 X5 | 2003 M3 | 2000 323 |2004 350Z | 2000 Porsche 996 | 1997 Prelude | 1989 300ZX | 1967 Mustang | 1970 Chevelle
Current Girlfriends:
2008 Expedition EL | 2008 Maxima SE | 2009 Shelby GT500 | 1966 Mustang GT