2011 X5 E70 twin turbo X5 35i

There is an odd whistling squeak or squeal under the hood at idle... most noticeable just at start-up and as the engine is turned of as well. Squeak... squeak... squeak... It goes away under power.

I suspect it has something to do with the turbo chargers but I am unable to locate the source. It may be something as simple as an air leak in one of the air hoses but all connections seem tight. It may also be metal-on-metal which is more a cause for concern, I'd expect. Maybe an impeller bearing or some such thing?

Anyone had a similar experience and found a solution?

Cheers.