"The Silent Downpipe"

  The sound of running water can be soothing, but the gurgle of water rushing thru the downpipes at night can be annoying and keep you awake. Thus...The Silent Downpipe!
  I envisage something that could be retro-fitted to existing pipes, but there would need to be versions for round and rectangular pipes. Or a totally new downpipe that can fit to new houses. Some understanding of the rotational direction of the water would be required. Anticlockwise in the southern hemisphere and reverse in the North.