Put ferrite beads or inductors in series with the motor terminals. Both terminals. Ground the case. Preferably, put small ceramic caps (10nF or so) across the motor leads, to the motor case, just outside the inductors/ferrite beads, to form an L-C filter.
Also put a current limiting resistor in series with pin 7, at least 100 ohms.
Note also the +10V needs to be well bypassed, both for 555 operation alone, and for the motor's changing load, and for the clamp diode to do its job. Typical would be 0.1uF at the 555; and >= 100uF (low ESR) at Q? and D1, all three located very close together.
Tim