looping "door knocking" effect

Hi,

I'd like to create the effect of the sound of the door keeps on knocking until the door is opened when anyone clicks on the door. I've tried using trigger, but not sure which one to pick so perhaps I thought maybe I need to use variables and I am not great with variables yet. Hope someone can assist so we all can learn from this example. 

Meanwhile I keep on trying at my end to create this effect.

Thanks.

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