howler monkeys

I've been thinking about a novel that involves an exorcism in the Amazon.
The first part of the book would be kinda like African Queen meets Heart of Darkness
with a priest, a female journalist, a native guide and a boat captain.

They will travel upriver and I picture at some point they arrive at the hut were the local Indian child is possessed and there are 6 of these howlers on top of it.

CAN YOU FEEL IT???