Hi Fiona and welcome to the Boards.
It is possible that your kneecap is not properly in the groove and is maltracking, causing this grinding noise, swelling and pain, you need to get your quads and surrounding muscles as strong as possible which will help with keeping the kneecap in place, however, this doesn't always correct the problem and if not you may need to return to your OS to have another look - I wouldn't leave it too long as if your kneecap is maltracking you're doing damage to your articular cartilage which can lead to arthritis. If in any doubt I would suggest seeing the OS again and having him check things over. Good luck.
Natalie.