WD40 was actually designed as a water displacer, not a lubricant (Water Displacing formula #40). The lubricants are relatively volatile, and are gone quickly. And before they evaporate, the components are one of the best dirt attractants out there. Use WD40 and it'll just make things worse. The only exception is to liberally WD40 the springs. Immediately go for a drive; if the sound is gone you're on the right track. I just bought an add a leaf for my pick-up, and the directions say to use a marine grade or other heavy grease on the anti-friction pads.
My experience is if you thoroughly grease the leaves, yes the grease holds dirt/ crud but on the OUTSIDE; the dirty crud may actually act as a seal keeping dirt and moisture out. For this to work, one would need to dis-assemble the spring pack and lube the interior parts. If you're in a wet environment, slathering grease on the outer portion may seal moisture on the INSIDE, and make matters worse. Take a close look at all the other components......could be steering, brake etc related.
If it isn't grown, it has to be mined