I notice this noise slightly all the time. There is a slight hiss, but I'm unable to determine the source. It becomes much more noticable when I change the position (accelerate or decelerate) of the gas pedal on a smooth section of hi-way. It also does the same thing when the cruise control is active. This was actually my first suspect, but it is able to maintain constant speed, so I've kinda rulled this out. The noise almost sounds like its coming from inside the car possibly on the passenger side underneath the dash, but this is mere speculation on my part. It sounds vaguely like a leaking damper that has to equalize itself once the state changes. Again, more speculation on my part.