After a while it falls and becomes a monotonous droning.

A monotonous droning noise caught her attention.

Smith's voice went on incessantly with its odd, monotonous droning, now loud, now soft, as he crossed and re-crossed the floor.

Vuvuzela -the bright colored plastic horns with a monotonous droning sound became the hallmark of the World Cup soccer tournament in South Africa.

The plastic horns became a hallmark of the World Cup in South Africa, producing a monotonous droning sound which provided a backdrop for every match.