I suppose it is because the exhaust gases squeezing through the narrow gap between where it has warped and the head make the noise.
If you are worried about this, look at it and you should be able to see soot marks where it is happening. Sadly, this is often accompanied by broken and/or lost studs and nuts.
In my case the gap between the warped bit and the head was very visible.
When I first heard it I thought it was an aux belt squealing.
Knowing the pain it was to fix, drilling out snapped off studs etc, I honestly hope it is your turbo, it'll be easier to fix.
Plenty of stuff on here about fixing the manifold though, should you need it.
This is why at LR shows you can buy modified manifolds with the webbing removed.