It is normal for some smoke alarms to sound briefly (up to 5-10 seconds) when they are powered up, especially older models. Newer models such as 9120B in most cases will remain silent upon power up. If the alarm continues to sound and no smoke is present, the cause may be:
Insufficient Battery or AC Power.
Very low batteries or insufficient electrical power (brown out) may cause a continuous weak sounding alarm. Temporarily disconnect power at the service panel until the brown out is over. If you do not restore the AC power, your smoke alarms cannot warn you of a fire.
Incompatible Warning Device If an incompatible alarm or auxiliary device is linked into a series of AC or AC/DC smoke alarms it may cause the system to alarm.