1985 Dodge Truck What is it, and what does it it do?

I have an 85 Dodge W350 with a 5.9L Federal engine. There's some kind of pump with a canister on top that's driven by the alternator belts. I don't know what it's purpose is. Bought the truck used, but there's a 3/4 rubber hose that comes out the side, and hooks up to the bottom of the unit. Other than that, isn't hooked up to anything else. Don't know if it was previously rerouted or bypassed. Making a lot of noise. Has a lid fastened with a wingnut. Canister is about 5 in deep by 6 in high with belt driven pump underneath. [img:9cd4947cfd]http://www.2carpros.com/forum/automotive_pictures/515107_Apr10_0001_1.jpg[/img:9cd4947cfd]