[blockquote]I have a 2006 S40 that has developed a whistling at Idle. If I take off the oil filler cap it goes away. It does it with or without air filter installed. Any ideas? [/blockquote]

 

go get your turbo looked at.....some times in the VW turbo jettas the turbo whistles when it is idling that is the sign that it is going out. not sure about the volvos....but that may be a good place to start.....hope that helps...haha

 

I've seen this so many times before.. By the sound of it.. Your car don't have a turbo.. if u take oil cap or dip stick out it releases the over vacuum of the crankcase ventilation system... Your PCV box or oil trap housing is clogged solid.... U need to replace that + the hose from the oil trap to valve cover have to be replace if u have the hard plastic hose and not the updated rubber hose... Sooner or later if u don't get that taken care of will set check engine light for Fuel trim codes... if it set have to fix the problem, erase codes and MAKE SURE DEALER RESET ADAPTATION (don't know how many time I have to say to do that) DENSO system are so sensitive and will set light again if adaptation not reset. It's pricey if u don't have warranty... but if u have make Volvo pay.