Well recently in driving I've heard a hissing noise upon hitting the gas, so today I opened the hood. I hear a hiss from somewhere between the exhaust manifold and the power steering fluid holder. When I listen it hisses and then stops for a second. When it hisses the engine rpms drop slightly and sounds as if it wants to stall and then the hiss stops and a second later will start back up. I can't find the leak much less do I know what to look for. any help is appreciated. Thanks