The noises or sensations you are feeling can be a number of things. It can be the knee replacement components, where you have friction. Also it's possible that it can be tendons, and scar tissue that has built up around the area or that was there even prior to surgery.
It is something many of those who have had total knee replacement surgery many years ago will sense.
It is not uncommon, as you still have an accumulation of fluid inside your knee capsule which is a combination of water, blood, and synovial fluid.
Did you see your hip surgeon? What did he say? Get a hip X-ray or CT scan to find out.