DC-9 Siren?

Question

what is that siren sound on the start up of dc-9's? 5 seconds after you hear the spool up, the siren starts and lasts like 15 seconds. So, what is it? If it's any help, the airline was Northwest.

Answer 1

possibility of the PTU  consult your questions in flightlevel350.com there i can proove to you my answer heard this myself in an Airbus aircraft dont know if it  is what you are asking :)

Answer 2

I fly NWA all the time....

Never herd a siren maybe what you herd was the APU??

Air New new zealand huh? Like your Aviator.