Hi, I have a volvo s40 2006 120k kilometees. I started to get a kind of whistle noise coming from the hood and my check engine light is on. The noise appear after about 5-8 minutes the car is driving and we only ear when the car is decelerating or stopped, at constant speed or acceleration the noise disappear. I changed my alternator and crankshaft strap and it's still there. Have any idea what it can be? Thank you.

 

It is near impossible to guess on something like that. You need to get your code read. You can also try to record the sound on your phone and post it somewhere like YouTube and post the link back here.

 

Does it stop if you remove the oil cap or dipstick?