First crack sounds a lot like breaking small sticks, it's more of a loud pop than a crack. The lid on your roaster could also shield the cracking from you as well. Second crack is more subtle, comparable to having a bowl of rice bubbles. More like quiet crackling. If she thought you were 100% crazy, try telling her you plan on roasting coffee in a popcorn popper, that really gets them thinking you should be in a mental institution.