S40 2011 Hissing sound when depressing clutch

[PierreC]

A friend with a 2011 S40 has a strange problem - there is a hissing sound (like the sound when the gas escapes a soda bottle being opened) when depressing the clutch pedal - and mostly when the car is cold (i.e. first thing in the morning for the first few miles).

Reservoir is full and it is NOT the groaning noise which would indicate a thrust or throw-out bearing problem.

Any ideas what may cause this hissing noise?

Thanks