Redfoot making grumbling noises

I've got a 13 year old tortoise that is eating well, active, growing and heavy. He sleeps in my bedroom and recently in the middle of the night he makes fairly loud grumbling noises, almost like a dinosaur or monster from a movie... I'm assuming it's just him digesting, but it's louder than my stomach when I'm hungry, so it seems weird. I've looked into all the noises a tortoise makes and it's not like any of the ones described online... Any thoughts?