Boost whistle noise when cold
 

2005 XC90 D5 auto. I have a hissing / whistling turbo boost related noise which I notice with a window down, but only at low speed for the first few miles of a journey. I've been telling myself that its a built in feature to reduce the boost pressure when the engine is cold, but suspect its more likely a leak which seals itself once everything is hot. Is there a common leaky joint? Is there an acknowledged way to find the leak which is only heard when the turbo is boosting? Or is it designed like this?

Check all the seals/joints for the turbo hoses inc both orange resonator box seals .... a weep of oil is usually the tell take to save dismantling, also check the condition of the intercooler.

Hope it helps.

Sounds like a boost leak, you will have to check all the hoses from the turbo and intercooler. Very good chance it is the resonator box orange seals that need to be replaced. Fairly straightforward for any garage to do.