What's that noise?

"Magnus the Arctic seal wakes up one early morning, puzzled by a strange, rumbling sound. What's that noise? Hare, Owl, and Polar Bear don't have any idea, but Walrus has a good suggestion: it's Magnus' rumbling tummy! After a yummy feast the friends all settle down to sleep--but suddenly there's another rumbling sound. What's that noise?"--Provided by publisher.