Answer
When we are hungry, any stimuli related to food, like smell, sight, or even thoughts, will cause our digestive system to send signals to our brain to prepare the body for food intake. In this case, the aroma of coffee activates the parasympathetic branch of his autonomic nervous system. And his body starts preparing for a meal, which causes salivation and activation of the stomach. The muscle lines of the stomach start moving and causes the rumbling sound.
Work Step by Step
Same as above.
