740 Source of intake/hose leak I can hear in cabin

So as the boost gauge reaches 0psi (remember: idle is vacuum in the manifold), I begin to hear a hissing sound inside the cabin, to the left side of the car. Since it reminds me of a hose leaking, I replaced the entire hose that goes to the boost gauge.

And the noise is still there. Thinking a gauge or a tee is leaking, I folded the hose and then clamped it just as it gets through the firewall into the cabin.


And the noise is still there. Next I clamped the hose that controls the flapper valve the left side of thecenter console/air ducts.

And the noise is still there. Finally I went to check any obvious leaks around the intake manifold even though the sound is not that loud. So far nothing.

Where else should I be looking at? Are there any hoses I may have missed?

Is this a +T car? Maybe put a one way check valve in line in the vacuum supply hose.

It was born as a turbo car. Hissing starts just before going to 0psi and continue as pressure increases.

By vacuum supply do you mean the line to the brake booster?

I think he means the one that sends vacuum to the HVAC. Have you noticed your defroster vents opening on their own, driving uphill and such?

Yes that's correct, the vacuum line that goes through the firewall to supply vacuum to the HVAC, you can get the check valves at any auto parts store. Typically black and white they only let air through one way. You'll want the airflow going towards the engine, you can blow through it to determine which way to install it.

Could be air escaping from the brake booster as well. Last failed booster I had on the wagon had a leak that you could hear just on light pedal, but if you have a failed check valve, you could be hearing that draining off as well.