In this text, ‘whistles like an old-fashioned kettle on the boil,’ is an example of a:

All right then, I say, so what about Dr Clark’s claim that "most severe ailments" have a connection with being dehydrated? Professor Hall leans back in his chair and whistles like an old-fashioned kettle on the boil. "If he’s trying to make the case that people are thirsty without knowing it, and if they just drank a little more they could sidestep everything from dementia to cancer, well, he better start making room on his mantelpiece for a Nobel prize. Maybe even two."