Definitions of grumbling:

• noun:   a complaint uttered in a low and indistinct tone
• noun:   a loud low dull continuous noise
• adjective:   continuous full and low-pitched throbbing sound
• adjective:   muttering discontentedly

  Example: "The grumbling boarders requested a meeting with the cook"

• name:  A surname (very rare: popularity rank in the U.S.: #72352)