What you need to know...
Firstly, it is important to know that cracks, pops and grating (known as crepitus) in your knees without pain is completely normal.
As of yet, there is no research to suggest a link between these noises and active pathology. It has been known for a long time that this is the case. One study by McCrea in 1987 found that 99% of people with 'normal knees' and no pain experienced crepitus (crunching, popping and grinding noises made within joints.)
What's happening to make the noises?
There are a few different theories. One suggests that it is tendons moving over bone as the joint moves. Another suggests that it is gas releasing as the capsule around the joint collapses which occurs naturally during movement (mainly traction like when you click your knuckles.)
The final theory is known as the slip-stick phenomenon where movement of the patella (kneecap) and the femur (thigh bone) produces a vibration which has been recorded on a vibration arthrography. The main outcome of some studies again found that crepitus is normal due to the knee having good, quality movement with high levels of joint lubricating fluid.
But I get fine grating sensations and not pops or clicks.
This is also normal as the fluid in the knee flows through the smaller spaces and crevices of the knee joint.
Knowing that when your knees make noises, as long as it is not painful, it means that your knees are healthy, well lubricated with good structural integrity.
If you do experience pain with these noises, do consult either your GP or osteopath.