Phhha
in which the h-sound is produced in the back of the throat and the 'a' is silent, is the sound one makes to emphasize how empty an action, reaction or situation is.
"When I ejaculate it feels like nothing comes out... Phhha..."
Person 1: "You are 5 minutes late."
Person 2: "Phhha..."
Person 1: "You are 5 minutes late."
Person 2: "Phhha..."