Progressive vs Linear Springs

Coil contact is the usual cause, but confirm it before you replace anything. With the car on the ground, have someone bounce the corner while you listen and feel at the spring, the top mount and the lower seat. Polished or bare patches between the tight coils confirm that the coils are touching. A worn spring seat, a dry top mount bearing or a loose anti-roll bar link makes a similar noise, and each is cheaper to fix than a spring.

A linear spring is usually the better choice for track use, because a constant rate keeps the handling balance the same at every point in the travel and makes damper tuning far simpler. Many lowering springs for road cars are progressive, which keeps some comfort over small bumps while firming up before the shorter travel runs out. For a road car that is driven hard only occasionally, that compromise is reasonable. If you plan to adjust corner weights or change dampers, it makes sense to start from a linear spring and choose its rate for the load at full bump.

Often it does, because the two ends may be wound or ground to suit different spring seats, and some manufacturers place the tight coils at a particular end. It is best to follow the marking on the spring or the drawing, and to check that each end sits fully in its seat before you load the spring.