when the engine is on, the hissing starts right at engine running and no matter what the rpms are , the hissing is still there. It is located on the drivers side near the rear above the valve heads. It will continued until the engines stop.

Sounds like a vacuum leak. Check the tee at the back for missing caps.

ya happened to me when i started it up once. it stalled shortly after and i got a crude comment from the chevy owner sittin next to me "wow starts real nice in a heated garage". a vacum line came off. one hose clamp later my problem was solved