However, the term crepitus has a wide variety of meanings, from the very benign to the very serious. So, if you look up the term, to not think that the sound you are having in the knee is one of the more significant ones, it most likely is not.
The crepitus in the knee is considered a benign condition.
In the vast majority of cases, the exact cause of it is not known. Just as the sound made when you crack your knuckles, the sounds coming from joints like the knee and ankle, are very common, and the cause is not known.
True, in some very specific situations, such as trauma, infection, etc, certain sounds can alert the doctor to specific diagnoses. In the past, the crepitus in the knee was thought to be due to roughening of the cartilage under the patella (kneecap). This idea is still put forth in many lay articles on joint problems. But, since the advent of the arthroscope, where the joint surfaces can be looked at directly, this has not been shown to be the case.
If fact, when the 'scope first came out, many procedures were done to shave down the cartilage under the patella to get rid of this sound, but it did not do anything, except make the cartilage thinner, so it wore out faster. Oops.
There is also a very common problem seen mostly in young adults, which is pain around and beneath the patella, called patellofemoral syndrome (PFS). It is also called retropatellar pain syndrome, anterior knee pain, and chondromalacia patella. It was thought that the crepitus was associated with this disorder, as many patients have both. But, again, it has been found, that there is no correlation between the two. Some patients have significant pain and significant sounds, but, there are also patients who have pain, without any crepitus at all. Then there are the patients, who have crepitus so loud that they can be heard walking down the hallway, but have no pain whatsoever. So, again, there is no correlation.
But, as stated earlier, there are certain sounds, other than the usual crackle, grinding, rubbing sound/sensation, that are associated with specific conditions.
A torn meniscus (cartilage) can have a sharp snap or pop, with severe pain, when a locked knee unlocks. A joint that is unstable, and it subluxes, will have a clunk when the joint relocates. There are many tendons that can snap over of bony prominence, such as the iliotibial band (ITB)snapping over the lateral condyle at the knee. Some degenerative conditions, can cause sounds in the joint, but these are almost always associated with pain.
So, in most cases of sounds within joints, the cause is not exactly known. And, even in cases where there may be a discrete cause, it is very difficult to ferret it out.
But, if the sound is bothering you significantly, you should see an orthopedic surgeon for an evaluation.