What the hell is it???

On my 99 F250 SD 7.4 PS, Auto trans. on the passenger side, under the starter selenoid, mounted on the fender well, there is some kind of a pump with what looks to be 2 vacuum hoses, one is piped to an oval shaped box next to the battery. A small black thing that is making a LOT of racket and vibration. It's driving me crazy. I need to relace it, but I don't know what it is. Is it crirical?
Thanks