I don't think it is an S&W, but that doesn't matter. The hammer or trigger is either badly worn or someone attempted to lighten the trigger pull. The result is that the pull is so light that the hammer is being released with almost no tension on the trigger finger, and the trigger is catching the hammer strut (double action sear) as the hammer falls. If the gun is an S&W, repairs might be possible, though I doubt parts would be available for a gun that old. If it is not an S&W, parts would simply not be available at all, and the gun cannot be repaired.
Note, of course, that the gun should still work in double action mode.
Jim