I have a thumping like behind my knee and i can see it thumping and feel it is that normal? 14 femal

Try : Try and see if the thumping is in time with your pulse, or if it appears to be from a muscle spasm. If you can see palpable pulse, that would be very concerning and you should be seen immediately. If you are having muscle spasms, you could try taking an anti-inflammatory and observe, but if it persists or you have pain or new symptoms then you should have a medical evaluation.