I have had better success rattling in the early part of the season than grunting. Then again, I turned a buck running with others using a grunt call early in the season, he ended up in the freezer. I don't use it much during the early season because I am observing their patterns more than anything. As it gets closer to the rut, that's when I start grunting.

Seems like if it's a noise they are familiar with, they will often investigate is my experience.