lex460l

i have probably the loudest ls i know of, 3 main noises i have noticed, 1 is a really annoying squeaking from somewhere in the front that they cant find, 2nd is the rear suspension that the dealer doesnt want to fix, and 3rd i finally concluded today is my passenger seat vibrating over bumps(a lot of bumps in nyc) i realized it doesnt happen when somebody is in the seat..... any advice on if theres a fix? has anybody had this issue as well?