I have an odd "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 as well as all of the rubber vacuum lines under the hood. 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 have also checked tightness of intake manifold bolts. Additionally (a planned project), I replaced the existing carb that was on the car when I bought it which was an Edelbrock 1406, 600cfm square bore mounted to an Edelbrock Olds 350 Performer intake manifold with a spread to square bore spacer with a rebuilt Rochester Q-Jet from UREMCO. Still have the "hissing" sound. I didn't think it had anything to do with the carb, but I figured I would mention the swap anyway. I have no idea what to do next. Does anyone have any suggestions?

New carb works great by the way.

 

I would check for a vacuum leak in the booster. Disconnect the vacuum line to the booster, pull a vacuum on it, and see if it holds vacuum. You may need something such as a MityVac to pull and measure vacuum.

 

Is there a hose from the engine going up to your air filter housing?
when I put a Holley on my 394 I had the hose hang down the side of the block as the original housing does not fit the Holley so I let the hose hand down and not connected anywhere and that’s what I was hearing. I have since put the Rochester back on connected the engine hose to the original air filter housing and now no hissing.

in my case the hose connects to the rear of the block between the heads.

 

Cap line going to booster and see if it stops hissing.

 

My money is on the seal between the master and booster.
It can be replaced without breaking open the lines if you have a gentle touch.

 

Chase all the suspect areas with WD40 or brake clean, if it is in fact a vacuum leak it will show up with an increase in RPM...Tedd

 

Yeah, that would be a lot easier than my suggestion.

 

I forgot about this post. I had the brake booster replaced, and not only did the hissing go away, the brakes work way better. Very little pedal pressure is required.

 

Good to hear that you fixed it.

 

Good feedback. Thanks for the update.

 

Glad the issue was resolved, and thank you for the closure.

My ride suffered from the same issue after 10 years.

 

Very important to close the loop, thanks.