means a continuous deep, resonant sound, like distant thunder, a hungry stomach, or heavy machinery in motion

from From 'rumble,' which surfaces in Middle English as 'romblen,' likely imitative — the word itself sounds like the low, rolling noise it names. It's a cousin of similar echoic words in Dutch ('rommelen') and German ('rummeln'), suggesting these languages all independently reached for much the same growl to capture a sound the body and the sky both make. The '-ing' simply turns the rumble into an ongoing event.