I have a "hissing" sound under the hood when the engine is running, but it stops when I put my foot on the brake. I thought it might be a vacuum leak to the brake booster, so I replaced the vacuum fitting for the brake booster on the intake manifold and the vacuum line (metal line and hose) to the booster. I also replaced the brake booster check valve. I still have the "hissing" sound after replacing the previously mentioned items. Brakes work fine. No issues with increased pedal pressure to stop the car. I've gone over everything under the hood to no avail. I checked tightness of intake manifold bolts and carb bolts. I have no idea what to do next. Does anyone have any suggestions?