My Volvo has developed a vacuum in its engine and when running make a loud rumble but when the Oil filler cap is removed the noise ceases.

Does anyone know what needs doing to make it stop rumbling and not develop a vacuum in the rocker cover?

 

first thing that comes to mind is the PCV oil separator (aka flame trap). What year is your car? on newer models its fairly easy to replace. Not sure if your car also has a vacuum pump that may be tied in that may have a wonky check valve so that may be worth googling about. Youtube has vids on how to work on the PCV system