Odds are the upper control arm bushings need to be lubed. This can be a chore on these cars because the grease fittings are impossible to get to unless the zerks have been replaced with 90 degree fittings. If you have ever seen a mustang with holes cut in the front of the shock towers that is why they are there. You could rent a spring compressor and then remove the spring followed by the upper control arms. Then remove the grease fittings, install 90 degree fittings so in the future you can grease it up easily. If someone already butchered your shock towers (I have never seen the holes cut nicely) then you should be able to grease them through the holes.