I have an 08 A/T, and the other day on start up, I noticed a slight hissing noise coming from the drivers side. I popped the hood and took off the brake fluid cover and it got exponentially louder. It makes the noise regardless of whether or not the air conditioner is on, and it makes the noise under idle, and accelerating conditions. It never goes away until the car is turned off. Anyone have an idea what this could be? I was thinking something with the air conditioner, but it makes the noise even when the AC is off. The SES light has never come on. Any help would be appreciated.

 

I've never encountered that issue myself but usually a hissing noise would suggest some sort of air leak. If it is by the brake reservoir it sounds like it could possible be a vacuum leak by the brake booster. I would suggest checking all of the hoses in that area to ensure that one isn't partially off or has a small hole in it.