drone on
To talk at length in a monotone. If you keep droning on like this, you're going to put your audience to sleep. A: "The professor's already been droning on about polynomials for an hour." B: "Oh, so that's why I lost interest an hour ago!"
drone on (about someone or something)
to lecture or narrate in a low-pitched, dull, and boring manner. The dull old professor droned on about Byron—or was it Keats? It was Shelley and, yes, he did drone on.