2003 Scenic - Hissing from servo into cabin...

[RussellF97]

2003 Scenic diesel Dci, When first starting the engine and when switching off, there's a hissing from the servo into the cabin. The brakes work fine, it's just the hissing that's concerning the owner...

[xantia_v6]

I don't know too much about the servo architecture on those, but I once experienced something similar on another car where a vacuum non-return valve got gummed up and stuck almost closed, making asthmatic noises at start up and shut down.