I don't know why, but I have the feeling that it's something the transmission simply does.
The 2004/2005 ones with the 5-speed Aisin Warner gearboxes seem to have this whine from 30-50mph, which seems to quiten down at higher speeds than that.
If you have ever taken a ride on a bus, a bus makes some sort of loud whine when going through the gears, up to 30-40mph. Other cars do this, one example being the BMW M3 which would whine at high speeds. As 30-50/60mph is high speeds for this large SUV, I have the feeling that maybe it's something the car just has. It is a large car with lots of sound deadening, so maybe its a safety system volvo implemented, for you to have a sense of speed? It hasn't seem to have got better or worse in our XC90 either...
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