I've worked in many a utility manhole and had to remove a black snake or 4 or possibly 5. I had 1 manhole where I'm sitting there splicing a cable and out darts a black snake out of one of the duct lines. It sees me and darts back in. 15 minutes later it comes back out and darts back in. We played that game for about 6-7 hours.
Black snakes are one of my least favorite snakes. When you pick them up they like to musk on you and it's a most foul odor.
Plug that duct...problem solved.