I had this same issue, the 2nd row single seat would squeak and rock back and forth a little bit. I have extended warranty and dealer said it was not covered and quoted me $2,500. I went on Craigslist and bought the whole 2nd row seats from someone parting out their QX60 for $250 and had the whole seat replaced by a mechanic for $50. I kept the old seat and took it apart to diagnose the issue and it came down to a bolt in the sliding track part that got a little loose.