Infection with the Lyme bacteria can cause this sort of internal vibration - a type of paresthesia (peripheral nerve sensation). When it's mild, you may first notice it at night when your body is quiet and still.
The supporting cast features James Caan as the young gambler, Charlene Holt, Ed Asner, Paul Fix, Arthur Hunnicutt, Michele Carey, and Christopher George. El Dorado was filmed on location in Tucson, Arizona and Kanab, Utah, and was shot in Technicolor. The paintings in the credits are by Olaf Wieghorst, who plays Swede Larsen in the film. The musical score was composed by Nelson Riddle.
Dorado is a constellation in the southern sky. It was named in the late 16th century and is now one of the 88 modern constellations. Its name refers to the dolphinfish, which is known as dorado in Spanish, although it has also been depicted as a swordfish.