There can be many different reasons why you feel it only on your left knee...
As you already mentioned, you have a leg length discrepency.
However, did you know leg length discrepency comes in 2 flavors: anatomical and functional.
Anatomical is the actual bone length. No one is exactly symetrical but above 1/4" can be felt by some patients.
Functional leg length discrepency is caused by a rotated and uneven innominate (pelvic bones.) If one half of the pelvic bone is somehow tilted forward, the socket of the thigh bone would drop a bit lower and functionally be longer. If you have this condition, you would also likely have lower back pain and sacroiliac pain as well.
Beyond leg length discrepency, the various joints in your feet can also have an affect on how the knee joint is loaded. The ankle is actuall 2 joints: The talocrual and calcaneal/subtalor. The subtalor can slide sideways too much, causing the entire ankle to lower. The mid foot nacivular joint can alos drop causing a pronated/flat foot, this will cause tilt and rotation of your tibia/shine boen and load your knee the wrong way as well.
Then there is the shape of the joint and the long bones involved. Are you knock kneed, bow legged, the list goes on.
Go see a GOOD PT and have a gait analysis done.
Some of the conditions are corrected by shoe wear, shoe lifts, orthotics, heel cups, bracing, training of proper running pattern, orif this problem is inthe pelvis, manual adjustments in conjunction to flexibility and strengthening of specific muscle groups (likely flexibility of the ITB & hip flexors and strengthening of the gluteous meds and VMO)...