In this case a person's knee like a runner ( yes I do that) can be a problem. The question you have to ask is when does it start? Does it get worse as the activity goes on? Does it get better as activity continues? Without any pain it becomes difficult to judge but generally with the knee it means some form of soft tissue damage. With that generally comes pain at some point. Somethings actually will improve as you run bursitis and plantar fasciitis can be that way. I had plantar fasciitis years ago on my left foot. It would hurt pretty bad starting to run and I would limp a bit but sure enough after about 1/2 mile it would go away and I could run fine. I then would creep back after running walking around but more I used it the better it tended to get. Doctor told me classic symptoms for some folks. After about 4 months it stopped happening and I have not had it since.
The knee one has to go back a see if they specifically did something to cause damage or tear. Sometimes it is subtle and you don't realize it then on re-checking you find you did something different. Simply hearing a cracking does not mean all that much until pain appears. I am jazz guitarist and I can crack my knuckles and making all kinds of popping sounds with my hands stretching them. This is simply joint noise from parts rubbing and sometimes air in the joints that is moving. Not necessarily an issue. My hands do not hurt doing this and it stretches the joints to keep them limber. I have no pain with it and play fine.
In the end as I said one should never take any information on the internet and especially this forum as medical information, Go to a doctor and get at least a professional answer. We all know that doctors do not always have any answer and they are simply going through the diagnosis part of medicine. Doctors have a license to practice medicine and they practice it but far from perfect. Does the knee hurt if the OP is riding out of the saddle? Does it hurt if they try and sprint from a hard gear or does it hurt and make noise spinning in high cadence.
In the end we must be our own advocates and see how far we can take information about our body to the doctor. I will stand by my premise that pain is the most important indicator and will tell you much. In the absence of pain then the disease is much more difficult to find and sometimes till it it too late. That is a real fact that is why some preventive test are done because at first no pain no symptoms. High blood pressure and heart disease are some of these types, as well as many types of cancer. At least a knee cracking noise while cycling is very specific so that hopefully gets pinpointed faster.