Engine Whistle When Warm

[v8alex]

I have a recent problem on my 2001 focus turbo diesel in that the engine has started to whistle. There is no noise when cold, but a low pitched whistle starts when at normal running temperature between 1500 and 2000 rpm which starts suddenly and increases in pitch then stops at 2000rpm. The car has no loss of power, economy etc so everything seems fine apart from the noise. It doesn't sound the real high pitched turbo whistle. Noise occurs both when mobile and stationary when free revving. Vid below which shows the problem:

Hope someone can help.

Alex