The sound of a knock on the window.

Six versions:

1. Knocking on Window 1 (0:01)

2. Knocking on Window 2 (0:02)

3. Knocking on Window 3 (0:02)

4. Knocking on Window 4 (0:02)

5. Knocking on Window 5 (0:01)

6. Knocking on Window 6 (0:01)

You get an archive with wav and mp3 files.