This particular article is purposefully "short and sweet" as the answer requires very little !!!!!
The knees often make a cracking noise called "crepitus".
This is caused by disturbances in soft tissue or a slightly misaligned knee/s.
Often, the sound is merely known as capitation, where the gap between the synovial fluid emits gas and makes a popping noise.
This is the same as when you crack your knuckles.
For those interested, see my earlier article on the knuckle cracking phenomena. Link below.
I may add that the only time to be concerned about cracking knees is when there is accompanying pain, in which case it may be caused by a cartilage problem, or uneven wear and tear.
If pain is accompanying the crepitus or capitation, it will be worth talking to your doctor about so relevant tests can be undertaken.
As an aside, and not wishing to complicate my answer, if the cracking knees are from a misaligned knee/s, it is usually worth talking to a practitioner who specialises in biomechanics and orthotics.
The reason for this is that knee misalignment generally comes from misalignment of foot posture eg when the feet are pronated or supinated.
If unchecked, this misalignment has a very good chance of turning into osteoarthritis of the knees as there becomes uneven wear on knee surfaces.
Dr Les Bailey,phd,DO,acopm,apta ( int part )
Les Bailey orthotics articles 2012
About Dr Les Bailey phd,DO, acopm.apta(int part)
Author,"laymans guide to foot and heel pain"
Direct phone number 07801418080
Dr Les Bailey began in physical therapies in 1981,qualifying in remedial massage,and later going on to qualify as an osteopath.
He gained his phd from OIUCM for a thesis on the treatment of plantar fasciitis.
He was awarded a teaching/lecturing diploma from the northern school of osteopaths in 1993.
He also holds a diploma in foot biomechanics .
Dr Les Bailey works from his clinic near Banstead/woodmansterne in surrey.