My popping noise would grow in intensity with lower temperature or increases in humidity. I discovered that my SPL upper control arms have a slight design deficiency. The space where the bearings sit are not machined right, or the inner spacer is not long enough. When the bolts that hold the arms on are properly torqued, they put excessive pressure on the inner races of the bearings. This causes a thrust bind in the bearing and rapid bearing failure. The symptom of this failure is a pop when the arm flexes, and/or excessive free play. I replaced the bearings and reduced the amount of torque on the mounting bolts. Haven't had the problem come up again. If it does, I will have to shim the spacer between the bearings to give proper support to the inner bearing races. Hope this helps.
