Bubbling in the coolant tank and a sweet smell. Combustion is getting where it should not.
A failing head gasket letting combustion gases into the cooling system.
Stop driving hot. A combustion leak test confirms it cheaply. Overheating finishes the job for you.
Reading about sounds only goes so far. The tuner on our home page listens in your browser and tells you what it hears. The app goes further: it names the exact fault, checks it against your vehicle, and tells you how urgent it is.