whoop sound in dc-9/md-80

Date:         04 Sep 99 11:38:36 
From:         EK <karijordet@hotmail.com>
Organization: University of Oslo, Norway
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I fly frequently with SAS' DC9s or MD80s. Sometimes during pushback while
the engines are starting one can hear an alarming "whoop whoop"sound over
the public address system. Is this sound intended for the cabin crew? If so,
what does it tell them?