Howling

I have two neutered male cats but every once in a while, late at night, one will emit a low guttural meow-type howl and repeat a couple of times.
Does anyone know what this type of howling means?
Occasionally, he sits near an open window and howls--again late at night.
He also usually gets very low to the ground with his head tilted up and back.
Cat brothers

My old male cat growled when another cat was in his territory.
He did a weird wailing when he was to engage in a fight or a cat in heat was wandering nearby.

Usually if a cat is howling at night near a window, I would suspect that there is a cat outside...or at least the scent of a cat that visited.

I have one cat who howls almost every night regardless of the situation. He's done it all of his life.

Hi Cat Brothers, welcome to TCS.

As CDubbie said, they can probably either see or smell another cat near the outside of your house and are warning them to leave!

My mothers cat does that. She is 18, I was wondering why she does it. Senility?