I had the same thing happen and mine then started a gurgling noise...

Mine turned out to be the sound of the vacuum pump dying on my 4.8iS so as a just in case watch out as when it went I lost 90% of my brakes but I also had to replace my booster master cylinder [emoji857]

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"When the Team Chief said.... You're trapped in a hole with nothing but a goat and a slinky, what do you do? Stubby said, I'm not sure but it won't end well for the goat...." ~(Overheard) Last day, Phase 3, Q Course