You may be describing what's called crepitus. This sound or feeling occurs when the back of the kneecap grates against the bone. This finding is more common before the ACL repair is done rather than afterwards.

Any snaps, clicks, crepitus, or joint sounds should be reported to the surgeon. The same goes for locking or giving way of the knee joint. Crepitus or similar sounds could occur when the extensor mechanism of the quadriceps muscle isn't working quite right. This can occur when scar tissue forms or if the tendon has been shortened too much.

The doctor will be able to tell the difference between sounds caused by scar tissue and those caused by cartilage rubbing against bone. There may be some treatment that can help you. It's not a normal sound and should be taken care of before it gets worse.