A medical device system comprising: a sensing circuit comprising: a first sensing channel configured to receive a first cardiac electrical signal via a first sensing electrode vector coupled to the medical device system and to sense R-waves in response to crossings of a sensing amplitude threshold by the first cardiac electrical signal, and a second sensing channel configured to receive a second cardiac electrical signal via a second sensing electrode vector coupled to the medical device system and different than the first sensing electrode vector; a memory; and a control circuit coupled to the sensing circuit and the memory, the control circuit configured to: store a time segment of the second cardiac electrical signal in the memory for each of the plurality of R-waves sensed by the first sensing channel; for each of a plurality of the stored time segments, determine a morphology parameter correlated to signal noise from the stored time segment; and detect the stored time segment as being a noisy signal segment based on the morphology parameter determined for the respective stored time segment; and withhold detection of a tachyarrhythmia episode in response to detecting at least a threshold number of the stored time segments as noisy signal segments. a sensing circuit comprising: a first sensing channel configured to receive a first cardiac electrical signal via a first sensing electrode vector coupled to the medical device system and to sense R-waves in response to crossings of a sensing amplitude threshold by the first cardiac electrical signal, and a second sensing channel configured to receive a second cardiac electrical signal via a second sensing electrode vector coupled to the medical device system and different than the first sensing electrode vector; a first sensing channel configured to receive a first cardiac electrical signal via a first sensing electrode vector coupled to the medical device system and to sense R-waves in response to crossings of a sensing amplitude threshold by the first cardiac electrical signal, and a second sensing channel configured to receive a second cardiac electrical signal via a second sensing electrode vector coupled to the medical device system and different than the first sensing electrode vector; a memory; and a control circuit coupled to the sensing circuit and the memory, the control circuit configured to: store a time segment of the second cardiac electrical signal in the memory for each of the plurality of R-waves sensed by the first sensing channel; for each of a plurality of the stored time segments, determine a morphology parameter correlated to signal noise from the stored time segment; and detect the stored time segment as being a noisy signal segment based on the morphology parameter determined for the respective stored time segment; and withhold detection of a tachyarrhythmia episode in response to detecting at least a threshold number of the stored time segments as noisy signal segments. store a time segment of the second cardiac electrical signal in the memory for each of the plurality of R-waves sensed by the first sensing channel; for each of a plurality of the stored time segments, determine a morphology parameter correlated to signal noise from the stored time segment; and detect the stored time segment as being a noisy signal segment based on the morphology parameter determined for the respective stored time segment; and determine a morphology parameter correlated to signal noise from the stored time segment; and detect the stored time segment as being a noisy signal segment based on the morphology parameter determined for the respective stored time segment; and withhold detection of a tachyarrhythmia episode in response to detecting at least a threshold number of the stored time segments as noisy signal segments.