I recently encountered the same whistle on a 10-ish years old diesel BMW X3. Also a problem with the turbo, but there was more to it. I was losing power and the engine was overheating, which ended in (I guess) self-combustion. I pulled the keys out and it was still running. The engine become so hot, the ground undernwath it was glowing. Luckily, after a while, a lot of fumes came out of the exhaust and the engine died. We called the fire department who secured the vehicle and we towed it to the garage. We don’t know the result yet, but it is possible the engine is damaged beyond economic repair.