1999 Ford Explorer rattling under the hood

Hi!!! When I start up my truck there is a rattling noise under the Hood I had my mechanic check it out and he Say's it's my pollution pump but idk what it is and would love to get this fixed and also my a/c works for a little while then blows out warm air how do I fix that and my gas gauge doesn't read properly how do I fix that also plz help is needed ASAP!!!! Thanks