Breaking a branch? The loud "crack" broadcasts a long way. I would think that any repeating noise. If I was listening for someone I would listen for un-natural sounds, like leaves crunching, but that does not carry far, maybe a quarter of a mile. You can make a whistle pretty easy though. A blade of grass or paper held between your thumbs, blow in the hollow (remember when you were a kid?) The cap of a sharpie or pretty much terminal tube. video of sharpie whistle Half cover with your thumb and hold at a 45 degree angle against your bottom lip. monkey squirt or helicopterfan held between your tongue and the roof of your mouth. (kid trick again)