When something starts to lose its intensity or fade away, we often use the phrase "guttering out." However, there are plenty of other synonyms you can use to describe this phenomenon. You could say something is petering out, dwindling, fading, or fizzling. Other options could be tapering off, waning, subsiding, or losing steam. Ultimately, it depends on the context and the tone you're going for. If you want to convey a sense of finality or disappointment, you might choose a stronger synonym like dying out or petering out. If you're trying to describe something more gradual and natural, a milder term like fizzling out could be more appropriate.