oric_dan(333):
Put the diode right at the motor. This reduces the inductive current-loop [motor
winding -> wires -> diode] to the minimum. Otherwise, it's like a big loop antenna,
broadcasting to the outside world.
Good point that I neglected, if the total distance was on the order of an inch the effect wouldn't be so bad. However, the OP did state the distance was about four feet away...