I have a 1998 Z28 A4. My car makes a strange noise, during acceleration and while coasting. From what I can tell it's coming from the engine. I'm no expert, it kind of sounds like something blowing air. It increases during acceleration. My car has also lost a lot of gas mileage, and lacks some power on the low end. All input appreciated. I do know I have a bad Front Knock Sensor, but doubt that has anything to do with it.
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1. vacuum leak
2. intake leak

Both are relatively easy to check.