Loud mysterious intermittent electronic noise!

On the road in my '92 Volvo 960 yesterday, I was STARTLED by a sudden booming noise that surrounded us and seemed to have no source. It sounded like lightning striking (weather was clear). It happened a couple of times, with no affect at all on the mechanical workings of the car, which continued to run perfectly well. This morning, it happened again when the trunk was open, and then again when a door was open. Neither time was the engine running. Thinking about it, the best way I can describe it is that it sounds electronic and "amplified," as if maybe it's coming through the car's speakers (though the radio was off in every case).

Thoughts?
 
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