The sensor still needs to be stabilized while shooting through the viewfinder. To save power, the SR mechanism isn't active unless the light meter is on (i.e. following a half-press).
If SR is turned off, the system is still powered up to keep the sensor locked in place. In other words, there's no way to get rid of the humming.
Originally posted by lightsource My K-3 does not do this
It does, but it's much harder to hear
Try a telephoto lens and you'll definitely hear it.
Adam
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