Lennox Furnace making ticking/knocking sound

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Moderator Post (s) for this thread: > The most usual cause by HKestenholz on 02/27/2007

I have a Lennox furnace that just started making this tick/kocking sound. I went to the attic and noticed there is a green light on the furnace and every couple of minutes it will start to flash and the ticking/knocking sound will start. This goes on for a few second and then stops and a few seconds later starts all over again. Does anyone know why this happens? What could be the cause of this? ANy help is greatly appreciated!