sasquatch

I have a 91 300se with many miles on it. I started it up this morning and the I was hearing this meat grinder like noise. Turns out there's a component on the pulley system just above the alternator that is making the noise. I inspected the pully sure enough the pulley was ****ed from the component, indicating thethe unit was shot.

My problem is that I'm not sure what this unit does or is called but it seems to be some sort of vacuum pump??? Can somebody help me out?

Diagram


Thanks