In embodiments a wearable cardioverter defibrillator (WCD) system is worn by an ambulatory patient. The WCD system analyzes an ECG signal of the patient, to determine whether or not the patient should be given an electric shock to restart their heart. If the WCD system determines that such a shock should be given, then it also determines whether or not a High Frequency (H-F) noise criterion is met by the ECG signal. If that H-F noise criterion is not met, the patient can be shocked. If, however, that H-F noise criterion is met, then the WCD system can confirm before shocking, by sensing another portion of the ECG signal, analyzing again, and so on. Thanks to the confirmation before shocking, the possibility is diminished that the ECG signal will indicate that a shock is needed falsely, due to H-F noise. This can further reduce false patient alarms, and so on.