Some people have a crunching (grating, cracking, popping) sound when they turn a head. Considering we're talking about long-term condition without any pain, is it considered a normal condition?
You're asking about so called articular release. It is considered a normal condition.
The sound, or the noise, is what people notice in articular release; the subjective relief it provides is secondary. (...) The sound generated by joint manipulation has been classified variously throughout osteopathic medical literature, being referred to as an “articular crack,” “articular pop,” “clunk,” “crepitus,” “joint click,” “snap,” “synovial grind,” and “thud,” and it has been described as a “grating” sound in the general medical literature (Figure 3). The articular release may be accompanied by a loud audible release or a soft joint sound—but it can also be inaudible. (...) The articular crack occurs for patients in both healthy and diseased states. It can be heard during normal functioning.
About long-term, habitual cracking:
Is articular release necessary to maintain joint health? (...) A person who undergoes habitual cracking does so for the feeling of relief and greater motion in the involved joint. If one were to consider the anatomic and physiologic models solely, one could assume that maintaining motion throughout the joint could lower the likelihood of developing osteoarthritis. On the other hand, the excessive use of a joint could lead to laxity of the ligaments supporting the joint, causing hypermobility or introducing an unnecessary stress that could eventually cause dysfunction.
There is nothing about the neck, but there is about knuckles, as researchers are generally more interested in investigating knuckle and hand cracking:
Swezey and Swezey studied the prevalence of knuckle cracking in geriatric men in comparison to 11-year-old children and found that their data failed to show that cracking leads to degenerative joint disease in the MCP joint in old age. The chief morbid consequence of habitual joint cracking appeared to be the annoyance inflicted on the casual observer.
If you're more interested in what exactly causes this strange sounds, I recommend reading "Real-Time Visualization of Joint Cavitation" (2015). There is a very interesting history section. But the final conclusion is:
Our data support the view that tribonucleation is the process which governs joint cracking. This process is characterized by rapid separation of surfaces with subsequent cavity formation, not bubble collapse as has been the prevailing viewpoint for more than a half century.
Its called Articular release!
Articular release is a physiologic event that may or may not be audible. It is seen in patients with healthy joints as well as those with somatic dysfunction. After an articular release, there is a difference in joint spacing-with the release increasing the distance between articular surfaces. Not all noise that emanates from a joint signifies an articular release.