Underneath the seat two pieces of seat frame come together. One is plastic, the other metal. The two go across the bottom of the seat across the seat. When they rub they squeak. Weather conditions affect (IMHO) level of squeak.

My tech at my dealer put a long piece of the fuzzy side of velcro between the metal and plastic pieces. The seat must be removed to do this. Both seats squeaked like jumping on an old bed. They never squeak anymore.

Article courtesy of: