A system, method, or device determines whether noise is present on a sampled and/or digitized sensed intrinsic cardiac signal based on a moving count of turning/inflection points of the signal. If noise is detected, the manner in which the cardiac signal is acquired, or the manner in which the device operates in response to the acquired cardiac signal (or both) is altered to reduce the risk of erroneously detecting noise as a heart depolarization and, therefore, inappropriately triggering or withholding therapy.