The sound of the "unstoppable round" effect triggering....And its not the stun sound. You'll hear a bit when you ADS for unstoppable, once it's an unstoppable round. It's THAT sound, but its like a skip in vinyl... over. And over. And over... and that's all you hear needless to say I muted that legend NF I ran... Didn't do it for unstoppable. Just Barrier and Overload. HOWEVER, with a pulse rifle, removing the AB mod, even having the US handcannon mod, ADS on the pulse rifle still creates this sound, only unequiping ALL champ mods gets it to go away.