I remember seeing a video before GT4 came out that showed them recording cars. I specifically remember some dood holding a mic by the exhaust of a yellow R33 Skyline (possibly a 400R). There was lots of on-track footage from Motegi, where they tested lots of cars, too; Kaz went out in a Nismo 350Z, I think - many of these cars didn't make it into the game.
I can't seem to find it. Anyway, there's a short bit in this "making of GT2" vid (!) and there is some footage I recognise from the test video I'm referring to in this GT4
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ending movie, too (0:34 onwards).
So it seems they record them up-close (as is only sensible, lest you get unwanted background colouration) and then use recordings of the cars on-track to fine-tune the sounds in-game (that's the part that was in the video I can't find.) Some of the cars we know were recorded on a dyno, the R8 for example.
Now, about that whirriness:
Source
The intake is that lighter grey plastic part at the very front of the engine bay, yet the single microphone in view is above the valve covers...
Thus none of that lumpy V12 meatiness makes it into the recording, just the sound of the valvetrain and anything else that might be closer than the heavily insulated / silenced intake sound. It won't matter if the car is on a dyno, either, which explains the R8... If all cars were recorded in the same way (hey, it's only one picture, maybe they did get the intake sound, I have no idea) then it's not fixable without going back to each car and recording the intake, too.
If it were me, I'd have taken the resonator off and stuck a mic between the two intake feed pipes, but it's not my car.
From
Jalopnik, on Forza's sounds:
I don't really like Forza's sound design of late, or its sound engine, but the samples are usually very good (at least until they get butchered by the artists).