The grinding could be a couple of things. First of all if it is a creeking sound/sensation more than a grinding it is a good posibility that you have some tendonitis (usually patellar tendonitis). Pain with this condition usually seems to be coming from the big tendon under the knee cap. If it is a actual "grinding" sensation with pain "under" your patella (knee cap) it is possible that it may be a condition called chondromalacia. This is a condition in which the underside of the patella, which is covered by a cartillage pad, is roughed up and when it rubs in between your femoral condyles (end of your femur near the knee) it causes a grinding and pain that feels like it is under the patella or "inside the knee. Usually it gets roughened by the the patella not tracking properly. This can be due to a weak quad muscle called the "VMO". THe condition is often referred to as patellofemoral pain syndrome. This can usually be taken care of by strengthening the "VMO". Another possibility is a condition involving the meniscus of the knee. This is more cartillage that helps cushion the knee. A problem with this usually involves pain along the inside of the knee. Sometimes a "locking" of the knee is associated with it after you perform a deep knee bend.
No matter what it is, if you are an avid climber and wish to get in some good climbs this spring/summer it would be a good idea to get it checked out by either a Physical Therapist, an Orthopaedic Surgeon, or an Athletic Trainer. Of course with out even looking at the knee or with out more info these are just educated guesses. But they may lead you in the right direction.
Good luck and good climbing
K Y L E