cardiffkid

Hi, I have a 2000 S6 with the 4.2 engine in it.Started running really bad yesterday, missing. Got home and ran the scanner on it for the check engine light it blinking on and off. showed three cylinders were misfiring.Checked under the hood and there is a round plastic vacuum valve at the rear of the engine right behind the manifold. It's sucking air like craze.Appears to have a small vent on the top where all the sucking is taking place.Large 3/4" vacuum hoses go to it.I believe it may go the the transmission.Can anyone tell me more about this valve? Looks easy to replace but I'm not really sure that is the whole problem. Ken