I think I fixed it, at least for now. I did more troubleshooting and narrowed it down to where the two vertical metal bars in the back of the seat connect to the seat frame through a hole. I couldn't see it well, but it felt like metal on metal. I spread a pea-sized dollop of lithium grease where they meet and everything is quiet now.I've had my 2018 EX-L for a little less than a month. I did not notice this when I test drove it, but the driver's seat squeaks in the lumbar area. Has anyone else experienced this and/or fixed it?