I have installed an external high current siren to my GC3 using RBSNTTL relay and 12v power supply. The siren works fine BUT....every time I arm or disarm the system the GC3 panel send 2 short loud bursts to the siren. This is normal on the inside control panel. I have been installing resistors across the siren terminals and I am up to 4.7k resistor. It doesn't have any affect on the sound burst. I used the following diagram for wiring.Click image for larger version. 

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