You could have a torn diaphram in the oil trap box. Quick test to check this is while the car idles remove the oil dipstick. You will hear a load suction and then the squeel should go away. Also the Check Engine light should be coming on soon if it has not come on yet posting codes for fuel trims. If this is your problem take it in to Volvo. The oil trap has an extended warranty for 10 years/150,000 mile warranty. Let me know what happens. Also if the oil trap does need to be replaced have the oil change as well while youre at it since the oil trap comes with a new oil filter installed already.