It was a famous FLUSH in my book.
As a former 'HDH'er I sighed mightily when the call left 850 AM. I think the toilet flush was an appropriate ending as things went. I wouldn't call the board-op a "cute", "wise-guy" as another message inferred. I think he was Really Feeling Feisty

. It was the last time the WHDH call letters were heard as legal ID on 850 AM (no it wasn't me, I left 'HDH in 1990).
Listening to the sign off (thanks for posting the mp3) makes me laugh but I also feel nostalgic. It was the sad end of an era. The record books have the board-op who did the "flush" as annonymous... but..
I know who did it, the former board op who was "on duty" and "flushed". He reads this board and probably will/would have identified himself if he wants/wanted to do so.
I won't "out him" here but salute him for a clever and timely piece of audio production which has become a legend. 8)