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.
El Nido is a small rural town in located 11 miles south of Merced, on California State Route 59. El Nido was founded early in the 20th century when the development of irrigation enabled intensified farming of the area.