The sound of the front door opening and closing a few seconds later was her answer.

Just then Eric heard the sound of the door opening again.

She looked up at the sound of the door opening.

There was definitely the sound of the front door opening.

The sound of the door opening caused us to turn.

She heard the sound of the back door opening and closing.

All heads turned at the sound of the door opening.

They both turned at the sound of the door opening.

At the sound of the door opening, he looked up.

When she came to, it was to the sound of the door opening.