Mutter and Rumble
Mutter verb - To express dissatisfaction, pain, or resentment usually tiresomely.
Rumble and mutter are semantically related in grumble topic. In some cases you can use "Rumble" instead the word "Mutter" as a verb or a noun, when it comes to topics like mumble.
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Google Ngram Viewer shows how "mutter" and "rumble" have occurred on timeline