Found a neat snapshot of the county Civil Defense siren system, from September 1972. Source is an unidentified newspaper article dated September 9, 1982. Likely the Fuquay-Varina Independent. 

The story is about the Fuquay-Varina Rural Fire Department having recently purchased and installed a new ten-horsepower emergency warning siren.

It was purchased with the aid of the Fuquay-Varina Civil Defense Office, and matching funds from the federal government, which paid half of the $3,000 cost. And had three emergency signals:

• Fire Signal – Series of rapidly alternating high and low-pitch tones.
  Hear example (either exact or similar) found on YouTube. 
  This alerted the volunteer firefighters. 
• Alert Warning – Steady three to five-minute sustained, double-tone blast.
  Hear example found on YouTube.
  Designed for alerting the public to a “possible threat to the general safety of the community.” This was a signal for residents to tune into local radio or television stations, for information on such situations from a tornado sighting to “strategic warning of an anticipating enemy attack.”
• Attack Warning – High- and low-note combination, changing from low to high volume, creating an wavering effect, for three to five minutes.
  Hear example found on YouTube.
  Used only for “warning of an actual enemy attack.” 

The article also includes a number of nifty historical points about the county-wide siren system in September 1972:

• Raleigh has five sirens.
• Cary has one siren.
• Garner has one siren.
• Holly Springs has one siren being installed.
• Wendell has one siren.
• Zebulon has one siren.

All have three-signal capabilities, like the new Fuquay-Varina siren. Nearby towns with similar sirens include Angier and other towns in Harnett County. 

All of the county sirens, and all three of their signals, can be activated “by radio beam” from the Raleigh/Wake County Emergency Communications Center (ECC). Following a siren activation, “county fire radio net” announcements are made to the appropriate fire department.

The ECC is in “constant contact” with the National Warning Center “via hotline,” and is monitored on a 24-hour basis. The alert and attack warning signals are tested on the first Wednesday of each month at 11:55 a.m., from the ECC