muttering under one's breath 低声咕哝
constant muttering 不断地嘀咕
stop muttering 停止喃喃自语
A: Why is he muttering like that?
A：他为什么那样嘀咕？
B: He is muttering because he is unhappy with the results.
B：他因为对结果不满意而正在咕哝。
1) 低声细语
2) 抱怨时的咕哝
3) 不清晰的喃喃自语
1. 咕哝，喃喃自语
present participle / noun
1. The act of speaking in a low, quiet, and often indistinct way.
Synonym: mumbling, murmuring