My car is a 1998 n-Turbo,the car runs fine,no codes,but I am having this issue.
when idling or driving slow and not on the Gas pedal,there is noise coming under the hood a whistling type of noise,as soon as I press the gas Pedal the noise goes away,it is not coming from the serpentine belt,for certain it is closer to the driver side where the canister is or that module that,s connected to the throttle body,not sure what it is called.

Anyone have a clue?

 

does it change when you pull out the dip stick or remove the oil fill cap? Google "volvo s70 vent box" for some similar posts. Also could be vacuum or intake leaks...

 

Thanks will give it a try when I get home.

 

Unfortunately,whether I remove the dipstick or Oil Cap,the Noise is still there,but there is Pressure for certain,only little Fume come out of the dipstick not a lot