Your spine should move up in a fairly straight line when doing abdominal exercises. The pop is probably harmless but you could still be using a less than perfect form.
When you lay flat on your back tilt you pelvis until the gap in the lower back area is completely gone ... that's a completely flat back. Doing crunches like the picture below is better for your back in the long run and pretty challenging. Just try to touch your knees with your elbows by lifting your entire upper body. Also when doing leg lifts pushing your back flat against the surface increases tension, you also need to contract your gluts to do so and slightly tilt your pelvis. Not saying you're not doing so already but i found this to be way more challenging and effective.