To make a continuous murmuring noise, often resembling the sound of a stream or brook. The brook burbled gently as it wound its way through the forest.
To speak in a confused or unintelligible way. He burbled on about his new project, but I couldn't understand a word he was saying.
Aeronautics: to experience or produce a separation of smooth airflow, resulting in turbulence over a surface. The wing began to burble as the aircraft approached its critical angle of attack.