I went to look outside and saw some very dark ominous clouds in the West. The sound came again, and from that direction, which confirmed that in fact thunder was rolling in.
About 5 minutes later, the storm hit. The grass up on the hill suddenly blew over flat to the ground with the strength of the wind, and the sheep went running to take shelter lower down in the gully.
The wind chime that normally hangs down from a branch was suspended in a horizontal position for quite some time as a solid gust of wind went whistling by. And then the cold sheets of icy hail began, and then lightning and thunder all around. The thunder rumbled down into the ground while the wind blew the hail against the windows.
It lasted for about 30 minutes before it finally turned to a calmer drizzle. But what an amazing storm it was. Out of nowhere. Like a piece of winter escaped and went wild with delight and blizzard-like madness before subsiding.