There are lots of structures in the knee which can and will make noises, sometimes horrible sounds, without any harm. There are of course also conditions where the noise should be of concern, for example if you have arthrosis. As long as I can remember my left knee made horrible noises but it did never hurt. That changed when I partly ruptured my ACL. But that is a different story. Some years later this noisy knee decided to bloc during some trekking tour in the swiss alpes. I had to limp two hours to my car, downhill in rough terrain with a knee I could not bent, and I thought that the knee was completely wrecked... I got it scoped and it turned out that there was absolutely no arthrosis (what the doctor suspected after hearing the sound of it) and nothing serious besides the ACL going on. Learning how to squat improved all conditions (noise and pain) remarkably well.
The takehome message is that noise can be a sign that something is wrong and it may be wise to let a doc check it. It is however more often than not (I suspect) not really serious. You should ask yourself:
When did it start? Is it angle dependant, in other words if you change the angle of your foot, turning it more in or out, during squats etc. does that have an effect on the noise? Is it more pronounced when the knee is cold, compared to after a workout? Things like that...