He could feel the anger 'boiling up' inside him. [=he could feel himself becoming very angry]
The water in the pot is 'boiling over'. = The pot (of water) is 'boiling over'.
The kettle (of water) is 'boiling'. = The water in the kettle is 'boiling'.
(..) Mr. Kelly, you have the most wonderful sense of humor. Hey, I own this joint And I know this guy And he is loaded. Mr. Kelly, we have your milk boiling just the way you like it back there for your steak. You're gonna like this. How's about I clear out the coat room... and you guys hit it in there (..)2009 TvShow: It's Always Sunny in Philadelphia Title: The Waitress Is Getting Married Season: 5 Episode 5