Then , the grand finale rumble ( or bottom burp , as Ian refers to it ) , went echoing first through my chamber , then , seconds later , through hers . I put my hand to my mouth in a dramatic gesture of embarrassment . Isla did the same . I giggled .

Question: What might Ian mean when he calls something a " bottom burp . "
Answer: It is something that is or sounds like flatulence .It is the sound a person makes when they are embarrassed and giggle .A bottom burp is something that goes through pipes quickly .It is a rumble of a car or some other large chambered vehicle .