Grumbling (noun)

1. A low, continuous sound of discontent or complaint.
2. Complaining or muttering in a low, discontented tone.

Origin:

From grumble + -ing.

Examples:

1. The grumbling of the stomach indicated that he was hungry.
2. The workers grumbling about the long hours.
3. He heard a low grumbling coming from the next room.
4. The grumbling of the thunder could be heard in the distance.
5. He was always grumbling about something or the other.