I take "ticking bomb" to mean a bomb that is counting down it's time till it detonates. It's used in the song to show how deadly hidden rage can be. Where we won't know when the rage bomb will detonate, and it could explode at any given moment. That someone could be hiding their rage so well, that one incident or one simple mishap of their day could be the moment that the bomb detonates. Whatever rage they're hiding, is the shadow that is buried and in turn silenced.
I take the song to mean, the things we don't want to talk about being repressed and buried so deep, to the point of silencing. It's so silent, yet it's a ticking bomb that is ready to explode, which leads to the conflict. The lyrics then go on to say "the raging storm below, will never go away, if it is not embraced." These shadows people hide deep down, need to be embraced before they explode and awaken the sleeping giant within. This is what needs to be done to spin that common thread, for our desire and ambition that there is good in this world.