antifreeze and diesels

From : Annonymous

Q: the a/c stops blowing thru the vents when accelerating anytime the engine is under heavier than normal load even going uphill. my truck is a 98 ram 5.2l with 88k miles on it. it seems like a vacuum operated vent is opening then closing with a thump sound when letting off the throttle. any ideas what the problem is thanks in advance js .