It is going to be very hard to determine the cause of a sound via text/typing on a forum to be honest.
With that said, is it your emergency tone (Settings>Sounds>Other Sounds>Emergency Tone)??
This just happened to me four times in a period of ten minutes at 4:30 in the morning. It's not an alarm and it shows no change of the display when it happens. It sounds like a carbon monoxide detector in volume and pitch... Very jarring to wake up to.