What sound did Samuel hear that exposed Saul's disobedience about the Amalekites?

Answer: Bleating sheep and lowing cattle

Saul claimed he had obeyed, but Samuel asked, "Then what is this bleating of sheep and lowing of cattle that I hear?"

“But Samuel replied, “Then what is this bleating of sheep and lowing of cattle that I hear?””— 1 Samuel 15:14 (BSB)