Complumbling
The act of mumbling complaints at an inaudible volume so as not to be heard by the subject of the complaint.
"What did she say when you told her you weren't going?"
"I don't know. I couldn't quite hear her, but I know she wasn't happy."
"How could you tell?"
"She was clearly complumbling."
"I don't know. I couldn't quite hear her, but I know she wasn't happy."
"How could you tell?"
"She was clearly complumbling."