Originally Posted by OptionZero

If the noise occurs during compression and sounds like a squeak, sometimes a sharp pop, that is the springs "binding" against metal.

The best i can explain it: when a spring is compressed down, the top and bottom rotate. thats physically the coiling/uncoiling of the spring. Well, what are the top and bottom touching? The upper mount and lower seat. All that stuff is metal. So you have metal moving against metal, which creates noise.

The radial bearing eliminates the noise by putting that mounting surface on a rotating platform with low friction. The thrust washer does the same thing - it's just two plastic (or i think one plastic and one metal) ring that goes between the spring and the mounting surface, so that rotation has less friction and therefore less noise.

The radial bearing is a sophisticated (and more expensive) solution.
The thrust washers are a cheap and just as practically effective solution.

I have used the swift washers on three cars (S14, IS300, Z34) and it has solved the spring popping noise every time.