mbmatt's response is excellent, but I would like to add one thing as food for thought. A hissing sound could be a serious vacuum leak, although you would probably not hear a vacuum leak while driving. In the case that it is indeed a vacuum leak a lean condition accompanies a vacuum leak. A lean condition is also a source of preignition.
As mbmatt said preignition is definitely not okay. It is to be repaired immediately. It will put a hole through the top of a piston. I've seen that happen.
I would put a vacuum guage on the engine and ensure that no vac leak exists.