My stomach rumbled loudly, reminding me I hadn't eaten
breakfast.
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Bumbled. Tumbled. Rumbled. Jumbled. Mumbled. Et cetera.
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S.E. Hinton
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"Rumbled" is a verb because it is an action word that describes the sound or movement made by something. It conveys the action of a deep, low, and continuous sound or the act of moving with a low, rumbling noise.
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This sentence doesn't have s+v+O word order so it is not easy to pick the s+v. If you rewrite the sentence:
A spontaneous demonstration rumbled from somewhere in the middle of the crowd.
The verb is rumbled, so ask your self - what rumbled? - and the answer is "A spontaneous demonstration". If the subject is a noun phrase then the simple subject is just the noun. In this case the noun is demonstration.
Probably the word order of the sentence is changed for effect.