A soft hum came from the interior of the ship.

We heard a soft hum and felt the elevator start to move.

He made no sound except for the soft hum of air through his spokes.

Then he heard the soft hum of Anna's voice and smiled.

We no longer saw him but could still hear his continual hum, loud or soft.

It simply hung in the air and sounded a soft musical hum.

The only sound in the cave was the soft hum of the power packs, which kept the lights on.

He let his words trail off with a soft hum.

The electricity was turned off and I missed the soft hum of machinery.

From the outside, you could hear only a soft hum.