Volvo XC 90 - whistling problem with waste valve on turbine -> I think.

A whistle is a hard thing to figure out, mainly because the tiniest hole can make a sound, even one that cannot be seen. I a a like problem with my intake manifold, I finally found it by carefully using starting fluid in small localized area along the manifold and when I found the hole the engine rpm came up then back down, but I could not see the hold even with a magnifier glass. So am not sure how to give you any ideas on how to check to find a hole you cannot see in your case, but I know even a antenna on your car can if bent slightly will also whistle on some models (this is why some antennas have a small wire wound around the main shaft, to break the wind passing the antenna.) Good luck on figuring this one out.