bourdonner

To buzz or hum, said of insects (une abeille qui bourdonne) or a low, steady mechanical sound (le moteur bourdonne). Also used for a crowd's murmur, and in the common idiom avoir les oreilles qui bourdonnent — to have ringing ears (or, figuratively, the sense that people are talking about you).

• Les abeilles bourdonnaient autour des fleurs. — The bees were buzzing around the flowers.
• Le réfrigérateur bourdonne toute la nuit. — The refrigerator hums all night long.
• J'ai les oreilles qui bourdonnent depuis le concert. — My ears have been ringing since the concert.

Don't mix up bourdonner (a continuous buzzing/humming sound) with vrombir, which describes a louder, more revving mechanical sound like an engine or a plane — bourdonner is softer and steadier.