Noise on a signal creates a triggering challenge for test equipment, especially oscilloscopes. Becuase the instrument itself also contributes noise, small signals in the millivolt range need proper instrument settings prevent noise from overwhelming the signal of interest. Even with larger-ampltude signals, noise can create a condition where a stable trigger is difficult to achieve.
Oscilloscope have built-in features to help deal with the noise. These features can sometimes be buried in menus, or not well known by infrequent oscilloscope users.
You should distinguish between simply suppressing and/or dealing with the displayed noise, and actually delivering a less noisy signal to the trigger circuit. Only the latter will create a stable trigger in these environments. Because oscilloscopes often route a small portion of the incoming electrical energy to a separate analog trigger circuit, any noise suppression techniques need to occur on the incoming signal, not the ADC processed or displayed signals. By triggering on post-ADC data, additional techniques for creating a stable trigger in noise become possible.
Common techniques for dealing with noise utilize averaging and/or using High Resolution mode. Averaging, which works on repetitive data only, is effective at combining data points from multiple acquisitions to reduce the displayed noise. Because this is a displayed data technique, it won’t suppress noise to the trigger circuit, and thus won’t create a stable trigger. Averaging won’t work on a single-shot event.
For more detail: View noisy signals with a stable oscilloscope trigger