Growling noises in Minecraft?

I started a Survival game on PC in Extreme Hills. I dug a shelter into the side of a cliff and during my first night I heard these growling noises. I didn't go to the window (a 1 block gap) in case it was a skeleton. Does anyone know what the noise is?

Every ussastanable sound is noice

That can be one of two things, a zombie, or a hostile wolf.

The both have distinct tones, so you shouldn't worry about them if you've bunkered in. They can be heard through walls

It's probably just a mob or if it is dark inside your shelter it's probably just the generic noises that they put in the game that play when you are in a dark area or cave.