Mutter

verb
  1. To speak or say something in a low or barely audible voice, especially in dissatisfaction or irritation. verb
    She muttered something under her breath as she walked away.
    He muttered a complaint about the long wait.
  2. A low or indistinct sound or utterance. noun
    There was a mutter of discontent from the crowd.
    The mutter of the engine could be heard in the distance.