Difficult question and it makes me hungry! Crunchy means that when you bite into something, there will be a noise. To think of crispy, think of fried foods. For example, "I bit into the crispy fried shrimp and it made a crunchy sound."
To crunch means that two things are pushed really closely together and make a noise. Crispy usually refers to food and means it's cooked to the opposite of soft.