This was around 6 years ago. I was on the Costa Brava, closer to the Pyrenees than Barcelona, in a duplex, part of a small group set into the steep slope of a wide, scalloped bay. I was sitting on a small terrace facing South East. Travel in a straight line from this point and you'd eventually hit Sicily, go left and you'd be heading for Marseilles.
The children had been put to bed and it was that time when you can settle and enjoy the evening before tiredness sets in. It was late May I think, warm at night, full moon halo mirrored in the Mediterranean, a slight breeze nudging the clouds.
As I sat there reading and looking at the sky I became aware of a low, deep drone. An engine I thought, maybe a ferry, perhaps a plane - but there was nothing in sight for miles. The noise expanded and amplified, seeming to emanate from the moon's reflection in the water, a smooth, gliding rumble pulsing outward in concentric circles and rebounding off the terraced inclines. The sound continued for about 2 minutes, growing in intensity and volume, then slowly faded, taking as long to dissipate, dissolving into the surrounding slopes, vegetation and structures.
Some acoustic trick I suppose, an aural brocken spectre, projecting an unseen sound, transforming the bay into a reverberant natural cathedral. Or maybe some escaped wild drone - a yawn of sea - or moonlight, augmented, magnified, and then made audible for a brief moment.