And the smooth hum of gliding progress, what had happened to that?

There was the low, smooth hum of perfect operation.

A car had rolled in through the driveway; its smooth hum marked it as a strange one, since most of the local automobiles were rattletraps.

He had heard that same smooth hum the night before, when a car had rolled into the shed beside the abandoned filling station.

He dozed off, then awoke, startled, as he heard the smooth hum of the lift platform rising to his level.

Cade's voice was a smooth hum of male satisfaction.

The engine turned over with a smooth hum.

It felt good to be out of the endless smooth hum of the main highway.

He opened up again as the air dropped behind, and the smooth hum of the atomics answered sweetly.

The smooth hum of his car's engine as he pulled the vehicle close by the side of the house was her answer.