What is the meaning of "bucket on the head" in this context?
Patulous Eustachian tube […] is a physical disorder where the Eustachian tube, which is normally closed, instead stays intermittently open. When this occurs, the patient experiences autophony, the hearing of self-generated sounds. These sounds, such as one's own breathing, voice, and heartbeat, vibrate directly onto the ear drum and can create a "bucket on the head" effect.
(Source)