Why a Thunderbird sounds like thunder wingspan, sighting in a dream

After being on a hot bus filled with students and helping the driver lower the windows in a dream, I was taken to a place where I was shown 3 legendary birds.  (I'm thankful to the one who found me worthy to see this).  2 were on the ground with me and I can't really remember what they looked like besides their feathers looking like raised oil paint and their snouts/beaks/heads being quite elongated.  They were also about as tall as me when they were standing on the ground normally. They were not reptiles, they were birds.  The colors weren't very out of the ordinary.  Flying above us was the Thunderbird, the king of birds.  The thunderbird was flying incredibly fast in a straight line across long distances, at least 100 miles an hour.  It was red, white, and blue.  One color was each on a part of the body.

I was taken to see a close-up of how it flys from behind it in slow motion.  I could see why its flight sounds like thunder, at the bottom of the wing's range of motion the wing flutters from the air, especially the bottom two joints (wing is 3 jointed like a finger).  This fluttering with a period of about 1/2 to 1/4 second in real time (hard to know because I was shown it in slow motion) causes the very loud sound  (I heard the sound!  It was amazing!) reminiscent of thunder.  The range of motion of the wing wasn't very much but the stroke was very powerful.  The wingtip probably raised a foot or two (each wing was about 6 ft long for a 12-14 foot wingspan roughly).  The bottom two joints didn't look too powered.  The top joint was curved and possibly has more joints in it because the curve became more compressed in the bottom of the powerstroke.  The compression of the first joint's curve, as well as slightly lowering where it is, is where the majority of the power comes from.  The fluttering at the bottom of the stroke (of primarily the bottom two joints and the bottom point of the "first" joint is where the thunder sound comes from.

From a side view the front side of the wing curled down during the power stroke in order to propel the bird forward and not just up.  Its like if your fingers were webbed and you pulled your first finger downwards when you flapped your  hands.  Also the whole wing tilted slightly forward during the power stroke.