grumble at (someone)
To complain or gripe to someone about someone or something. I turned the thermostat up, so you all can stop grumbling at me about how cold it is in here, thanks. Of course Marjorie is grumbling at anyone who will listen about how I left dishes out in the kitchen—she's not happy unless everything is perfectly put away.
grumble at someone
to complain to someone. Go grumble at someone else. I'm tired of listening. Stop grumbling at me!