kf2qd:
try this pattern - 1000 0010 0100 0001Arduino pins 2 & 3 are controlling 1 winding. pins 3 and 4 are controlling the other winding. So the motor is buzzing becase you start in one direction with pin 2 and then immediately reverse it with pin 3.
OR - try wiring it this way - Arduino -2 to LM293 -2, Arduino -3 to LM293 -10, Arduino -4 to LM293 -7, Arduino -5 to LM293 -15, and use your original output pattern.
Good rule of thumb - If a stepper motor vibrates, it is because you are not powering the phases in proper sequence. The power sequence for a bipolar motor in simple stepping looks like this - A+ B+ A- B-, you wiring has A+ A- B+ B- You need to alternate with winding A powered and then winding B powered.