Your assessment of the situation is the same as mine. I could hear but could not narrow down the location. I could hear it coming from the top of the engine. After removing the engine cover I clearly hear where the hissing sound was generating. But, I could not remove that long black rectangular cover to have full access to the hissing sound. It appears thag rectangular box covers all sorts of wiring. May just need to take a ride to my friend's shop to have all the proper tools to work.