Smell through windows

[grounch]

Hi there

It was more an issue in summer when driving with open windows, but still buging me now so i'm seeking advices.

When i drive windows open, and put a bit of load on the engine (climbing a hill fo example) i can smell fumes or some kind of "metal heat smell" inside the car. The last one is more noticeable when flooring it.

My first guess would have been the exhaust manifold as they often crack on this car (it's a 2005 3.0d btw). But that issue includes fumes under the hood, and smell inside the car by the ventilation. And i absolutely don't have those 2 symptoms. Windows closed it never smells even a little.

Did not checked for cracks because of the poor access there, even with a snake camera, but i'm pretty sure it's not that.
What else could be causing this annoyance?

As other symptom i have a turbo whistle on low rpm range (until 2.5 3k rpm).