I am a little late to the party. I own a 2014 Lexus IS250 AWD, recently I have noticed the same rattling sound coming from the passenger seat. I did a little digging and found it was the guide rail making that noise as Rabbers mentioned (see attached video). I am going to try put some WD-40 to see if it solves the problem. It does seem like a temperature related issue as it started to occur this winter when it got very cold.
Lexus IS250 front seat rattle.mov