What string are you talking about?
If you notice a ball attached to the end of a stringy kind of growth, it could be a normal testicular wart. Consult a dermatologist to get rid of it through a painless, easy procedure.
Otherwise it could be cancer. Please go to a reputed hospital and ask for a check up. If the doctor thinks it's cancer, he'll do a biopsy of the ball. Basically, he'll take a small piece and send it for testing. It's a very quick process. After that you will know if you have cancer or not.