Chattering noise from back of motor

[Chevyman680063]

I was driving the truck around this past Sunday and it start making a noise from the back of motor/front of transmission area. It is still making that noise and it does it as soon as I crank it up. It sounds louder when it's in park and neutral. You can hear it when driving as well but only as you're slowing down or start moving b/c the sound of the motor drowns it out. The motor and trans. are 350/350. Here's a video I made of it earlier today. Thanks for the help!
https://www.youtube.com/watch?v=72JPwKZ5x8I