That loud electrical hum is your auditory system idling without external input while your brain is in a hyper-alert state.
Quick facts
Typical length20 seconds - 2 minutes
Ends by itselfAlways, without exception
Physical harmNone documented
People affectedAbout 8% at least once
Many people describe an intense ringing, buzzing, static or a sound like a train approaching. It is one of the most common sensory features and one of the most reliable early warnings.
Key idea
During the REM-wake overlap, the brainstem is highly activated while the ears receive no meaningful sound. The auditory cortex amplifies its own internal noise floor, and the alarm state makes it feel enormous and threatening.
This is closely related to 'exploding head syndrome', a harmless phenomenon where people hear a loud bang at sleep onset.
Reframing helps enormously: the sound is a timestamp. It tells you an episode is starting, which means you can start your coping sequence immediately.
The sound is internal, not in the room.
It is your earliest warning signal.
Use it to start micro-movements.
Sources & further reading
Links to peer-reviewed research and recognised medical organisations.