WD40 isn't a permanent lube. White lithium grease would do much better in metal to metal contact. Is it metal on metal or a bushing kind of squeak? Because the control arm bushings and ball joints are upto 11 years old and warn in on one location, it's probably normal for there to be noises. You might try greasing the ball joints and spraying a silicone grease into the bushings to see if it helps. Odd are though, the only cure is new bushings.