Ibn al Qayyim said in Zaad al Ma'aad:

"As for the weeping of the Prophet (saw) it was in the same degree as his laughter. He wouldn't sob loudly and raise his voice, just like his laughter wasn't loud. However his eyes would fill up with tears, until they flowed out, and you would hear the sound like that of a whistling kettle coming from his chest. He would weep out of mercy for the dead, out of fear and compassion for his ummah, out of deep fear of Allah, upon listening to the Qur'an. And it was a weeping of longing, love and exaltation, accompanied by fear and khashyah".