It's creppy, I mean certain sounds are often recorded at Demonic places, but it could be Micheal, it's GTA V, things are more advanced, it's a 50/50% chance, but still good work, this is a mystery.

If you go down there and shoot rats and kill them, an odd sound can be heard in the background.

Sounds like a mouse or rat...dunno,my mind can also play with me.

Good find.

Ali Rocky you know what you said about the sounds being recorded at Demonic places R* could have easily edited in the sounds that could be ratman but i am not saying that R* did 

In this maintenace tunnels you can hear a lot of strange noises,I investigated it a lot of times and I haven't found nothing beyond misteyous noises. It's really weird

Honestly.  All it is the gust of wind formed by the heat extractors and the cold front of the water sewage flowing and combining, it's just a sound effect. 

Could just be sounds from the machinery

I'm not familiar with sounds in the tunnels, but I can tell you that definitely is not footsteps unless whatever is making them has something like 6 legs and is galloping along. The number of steps taken in the 2 seconds we hear the sound is between 8 and 10, depending on what you classify an echo.

The sound is stereophonic, as indicated around the 17 second mark when the final sounds shift to the left channel when the camera is moved around Michael, appearing to be originating from the East.

The sound is comparable to a wet dog shaking it’s coat of water, but sped up and pitched several semi-tones higher. It sounds almost like some kind of weird Avery vocalization. It could very well be a glitch relating to the rats in the Tunnels. In this case, a rat could have partially clipped through the tunnel wall or floor perhaps and sustained repeated damage and injury to it’s body in very quick succession, causing a fast meaty/clucky sound.

It was good idea to follow the rats! They are most likely to lead us to RatMan!