I get the same thing from time to time. Always goes away when pressure is put on the seat (someones in it). Wondering if it couldn't be the rails underneath. The seat motion always feels sloppy when I move it. Never really checked though.
'98 Contour LX-White
2.5L ATX
94,000 miles
KKM Tru-Rev / FMS Wires
19mm BAT rear swaybar