took it to ford 4 times and they did all those tsb's for hum,vibrations and still hasn't fixed it.they call the ford tech's and said all 2005 f-150's exhausts are made from stainless steel and don't flex causing a exhaust hum behind the drivers seat and though the truck at speeds from 50-60 mph with going up a slight incline .i think it's a cop out and they don't want to really fix it.so i need to know if anyone has or had this problem and if they repaired it.thanks steve