Every time that I have heard a popping noise like you describe, I have found that my frame has cracked. Look at your steering gear while someone turns the wheel and see if it flexes. There are 4 bolts that hold it to the frame, Make sure they are tight and look closely at the frame around those bolt holes. also look at the bumper bracket on the drivers side, look at the frame where the shocks mount
Bushings will usually rattle or you will have a hard knock
also look at the cross member by the steering box.