// outside of the class, below "Using UnityEngine"
using Random = UnityEngine.Random;

// make sure to initialise this properly
fudgeFactor = 30;

// inside the update function
atan2 = Mathf.Atan2 (v_diff.y, v_diff.x);
Quaternion newAngle = Quaternion.Euler (0f, 0f, atan2 * Mathf.Rad2Deg + Random.Range( - fudgeFactor, fudgeFactor) );