I drive some very rough roads so, perhaps this will not happen to most owners...
My Mercedes Sprinter had been making a whistling sound from the turbo area for about a year. I had no luck in figuring it out until just recently, when changing the oil. I even mentioned it to the dealer, who just shrugged and no mechanical inspection found it.
What I found is that the turbo inlet tube in the engine compartment (top right near front grill area) had popped up and out of the socket into which it fits. It was sitting slightly ajar on top of the hole into which it should be inserted, allowing air to bypass the inlet in the grill area, and to enter directly into the main tube leading to the turbo.
The whistling noise disappeared once I fitted the front inlet piece into the tube.
I don’t know where the filter is for incoming air, but I hope it is after this juncture, otherwise dirty/dusty air would have been directly entering the turbo, doing who knows what to the turbo internals.
Lesson learned: familiarize yourself with the engine compartment. Look for stuff like this turbo tube popping out/up, rat pee and poop and/or damage/chewing, anything that looks different from the last look-see.