I think hogs rallying is one of the best sounds ever. I can think of a couple times where I have been hunting and like uglydog said you hear a bay and then you start hearing the chatter and then the chatter starts sounding like thunder with grunts and you cant hear a thing but hogs! It is a neat feeling, one that you don't soon forget or get to hear often enough.
I've only had them rally on me one time, I had no clue what in the hell that sound was once my CD got there.
It was a cool sight to see them bunched up, just a waiting for something to try them....
