Loud whistling noise coming after 2500 rpm

My friend Defender TD5 2001 model have weird problem, please help.

Loud whistling noise coming after 2500 rpm, it looks it may be from turbo as when he has driven without turbo there was no noise.

But on the other side when he swap with new turbo (as a test) still sound comes after 2500 rpm or 0.5 bar.

Any educated guesses please????

Thanks in advance.