A system including a means for estimating the residual noise level in electrocardiogram (ECG) signals is disclosed. The disclosed system and methods may be used in an electrocardiograph devices. According to an exemplary embodiment of the present invention, a plurality of electrodes positioned in proximity to a cardiac structure may measure an electrical signal of the cardiac structure to produce the ECG signal. The system may segment the ECG signal into a plurality of segments. For each of the plurality of segments, the linear trend energy and/or direct current (DC) energy may be removed from the segment, and the estimated noise energy of the segment may be calculated. A subset of the plurality of segments with a minimum estimated noise energy may be selected. The residual noise energy of the ECG signal may be estimated by calculating an average of the estimated noise energy over the subset of segments.