If one testucule feels larger and harder for a 43 years old what could it be? doctor asked for ultrasound,pet scan was all clear 7 months ago.

Can be orchitis: infection or inflammation of the testicle, hydrocele or varicocele- I agree on the need for testicular ultrasound.

Lump: If you feel a lump in your testicle you should speak with your doctor to get checked for testicular cancer. http://www.webmd.com/men/guide/testicular-examination-and-testicular-self-examination-tse#1

Probably nothing: It is normal for one testicle to be larger than the other. An ultrasound sounds like the right test for you at this point.