Howlers are so named because they often, especially when they feel threatened, join in a chorus, by the whole troop, to frighten the "intruders" away. The sound that I heard was more like a Hooooo Hoooooo from deep in the throat. It was not uncommon to hear them just before daybreak in one of these choruses. They move more slowly and use their tails more for support and hanging as they move between branches than the white-faced capuchin monkeys do.