Most likely a ligament that goes "pop" just like popping your knuckle. Without swelling, you probably didn't hurt anything, but if you can get your insurance to spring for an MRI, that isn't a bad idea to have a look at it. I got clipped real badly playing football and likely tore my meniscus, but didn't get it checked out. Five years ago deer hunting with a too heavy a pack my right knee went over. It swelled up really badly and getting back to the truck was a chore. Partially torn ACL and when the doc fixed it he said that the old torn meniscus pieces were wearing on the good parts and he had to take out a bunch of the meniscus. Don't let that happen to you. The doc said that a new knee would probably be in my future. You just have so many miles left in your knees. Be smart about how you are using them up. I'd sure think that by now, someone would have come up with a synthetic or cadaver replacement for a meniscus.
