Checking the version

WARNING: This page is about a different car, the 2004 Volvo S40, 2003 Volvo S40, 2002 Volvo S40, 2001 Volvo S40, and 2000 Volvo S40. However, it is still accessible from the selected car via links, so may be relevant.

Fig 1: Identifying Hissing Sound From Oil Filler Opening

Courtesy of VOLVO CARS OF NORTH AMERICA.

Adjust the pressure regulator to a maximum pressure of 0.3 bar
A gentle hissing sound may be heard from the oil filler opening. This is because an intake valve is open and cylinder pressure is leaking past the piston rings.

Check the components and connections. See Checking Air And Vacuum Hoses, Turbocharged Engines
Apply soap solution to the connections if leakage is suspected
Bubbles are permitted around the throttle spindle
Small bubbles are permitted between throttle body (TB) and the intake hose (if the hose is made of plastic). There must not be any bubbles if the hose is made of rubber
Checking for air leakage from the canister purge (CP) valve. Leakage is not permitted.