Re: if the knee makes a grinding feeling and not treated what will happen
You probably have something called chondromalacia of the patella(kneecap). It is a flaking off of the cartilage on the back of the kneecap. It has different causes but the most common is a kneecap that doesn't sit properly on the front of the knee and may go more sideways than it should when you walk.
The treatment for it is to correct the cause but if you already have significant arthritis(that is what arthritis does...makes cartilage flake off)then it may be too late to treat. It would need to be evaluated by an orthopedic knee specialist. But eventually you'll end up with no cartilage on the back of the kneecap and the pain will increase. The pain in the back of the knee is because of joint swelling from all this inflammation and because the knee tends to push those bits of ground up cartilage into a pouch on the back of the knee. They call it a Baker's Cyst but it's actually a bursa. It will go down when the arthritis is gone.
As this cartilage is shed by your kneecap, it will also inflame the entire knee joint and may cause arthritis to settle in there as well.
You didn't say how old you are but if you are relatively young, get it looked at and see if it can be corrected. If you are older, you still need to have it looked at and to have the entire joint evaluated. I had it in both knees and had my kneecaps "capped" with plastic when I had my knees replaced.
We depend on our knees to get around and they are nothing to mess around with. Take it from someone who was walking with 2 canes in her 30's while raising kids....very hard and painful. When your knees are messed up, your life is messed up. I had both replaced at 47.
Please, for the sake of your future, see an orthopedic knee specialist.