it is cast upon yourself, and it goes where you go and lasts long enoough,
it is good for mobs that are aggro, for example
if you were lvl 30 and normally went to the chickens (Kokuru) outside samarkand and ran past one it would begin to attack you
with noise it would not even notice you, (unless it was a champion or giant)
this is very helpful for training alone, you can attack one mob at a time without worrying about 3 more ganging up on you at the same time
I hope this helps some more with what has been said already
