WELL WELL WELL! What do we have here??..
Seems PO or PM may have skimped during service or perhaps even this turbo was used for parts!
There is only ONE of the usually three braces that hold the turbo together, and thus there are only TWO of the usually six bolts. One of them is almost falling out!
Also plenty of soot around it. I'm going to go ahead and say this may be the reason the turbo whistle's so loudly. Clearly there is not a great seal going on here.
I've been getting around 9 to 9.5 PSI on the boost pressure readings during drives, so certainly it's only having a partial impact. On my previous (refurbed) turbo i was getting around 12. It was also not whistling.

I'm not 100% convinced this is the only source of exhaust getting into cabin, however. But maybe. It's a good bit further away from firewall than where i thought i've occasionally seen exhaust coming up from engine in the recent past. But that could have very well been just a breeze tossing it back.

I will need to either order the other two brackets or just pull off the brackets from my good turbo in other vehicle. Problem is i can't tell how easy it's going to be to get that one bolt back in place.