Marshall Efron only voiced a single Transformers character, who didn't appear in too many episodes right at the end of the original series cartoon - Hun-grrr (along with another voice actor, according to TFwiki, so they may have shared the role over different episodes, or one did the beast voice(s) while the other did the robot voice).
According to the New York Times, he died of a cardiac arrest on September 30th at the age of 81.