What causes throbbing in the testicles?

Many things. Many things can lead to achiness, even pain. Of course injury is one cause. Men can develop low grade infections in the testicle, epididymis causing pain. A prostate infection is common as well. Other causes include hernia, and varicocele. Occasionally a mass/tumor can be the cause. A careful examination of the testicles, groin and lower abdomen is needed to better determine the cause.