Magnitude of acceleration vector not 0 at rest

I want to rectify ADXL345 readings so they are only positive and based on 0 when at rest, but the value of mAV fluctuates between 0.31 and 0.47 although the set-up sits on a concrete basement floor, with no vibrating machines or vehicles nearby. Is this some kind of noise?

const float valueShift = 9.81; 
mAV = abs(sqrt(ax * ax + ay * ay + az * az) - valueShift);

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