To make a continuous low sound. (Cambridge Dictionary)
To make a low heavy rolling sound. (Merriam – Webster)
To travel with a low reverberating sound. (Merriam -Webster)
To speak in a low rolling tone. (Merriam – Webster)
To engage in a rumble. (Merriam – Webster)
Make a continuous deep, resonant sound. (Oxford Dictionaries)
A rumble is a low continuous noise. (Collins Dictionaries)
If a vehicle rumbles somewhere, it moves slowly forward while making a low continuous noise. (Collins Dictionary)
If you refer to the rumble of someone’s voice, you mean their voice sounds very low, making it hard to hear exactly what they are saying. (Collins Dictionary)
Words mumble and rumble.
Why Thunder Rumbles. Weather Online.