i have a 00 bravada with 78k. recently i've noticed that only while the engine is running, there is a variable "woosh", (like shhhhhhhhh...) or possible vaccuum sound coming from down near the pedals. it changes as i accelerate or coast. when i turn the engine off, it does a short "woosh", then stops, even with the key still on. does anyone know what this is and how to fix it? thanks.