Kumho AT52. Much quieter than the mud pluggers the last own had on mine. Wearing very well. Good in the dry and wet. Only driven on grass and gravel as "offroading" goes and they were absolutely fine.
I suppose the only real way is check your impeller against an original and see if they differ. From what I've seen it's the angle of the vanes that get's the whistle. Alternatively, take it to a turbo specialist and see what they say.
You can actually buy aftermarket impellers specifically to get a whistle. Quite the thing in the US pickup community
Check your impeller isn't chipped or bent, maybe? That seems to be where it originates from.
If it was a mechanical diesel; isuzu, 200tdi, etc, I wouldn't worry about it. A TD5 and later aren't as robust. I'd be inclined to follow miktdish's idea. Drain some and top up.
injector/pump seals are fine, they'll be nitrile or viton.
Some use what miktdish posted. I prefer a wire wheel on an angle grinder
https://www.screwfix.com/p/titan-multi-material-wire-wheel-brush-100mm-3-93-/239PR?tc=AB8&gStoreCode=AB8&gQT=1
I know you cant take the body off to check some of those places, but you should do your best to check them...
Odd, as my Isuzu 2.8 sits fine without modding the bonnet. I fitted the top mount intercooler and it still missed the bonnet, though I cut a hole anyway for the bonnet scoop.
How do you know you're down on power? Because it's noticable, or because a Rolling Road told you?
If a RR told you it was 190, did the exact same RR tell you it's now 160? You can get VASTLY different results from RR to RR.
Do you really? I thought it would work as good sound deadening. My son used it on his camper, hence why I posted it. Not sure of the sound deadening properties, personally.