This cars sound is weird

[Vertigo]

https://www.youtube.com/watch?v=7Xz6DcDs8lc

What gives it that warbling effect?

[Dell'Orto]

My guess would be manifold design, and maybe antilag?

[GDII]

I watched that video and then watched the same car on snow in Sweden. The snow video lacked the warble you can hear on gravel. This sounds like a stupid idea but on gravel cars seem to grip and lose grip very rapidly which causes corrugations in the road. The plume of gravel being throw out the back comes up in bursts rather than a stead stream.

I don't think this is the case but it could be. Does that car sound the same on tarmac? I haven't watched any videos on that.

[Flannelman]

It could be a lot of things, but I think that warble is now the generic sound on WRC cars. The only thing that all engines in common is the 300hp restriction plate on the turbo entry which may be the main cause of this sound.

I presume that when the impeller side of the turbo exceeds the restricor plate, a vacuum is created between it. This reduces boost pressure, which goes to reduced combustion pressure, less pressure out the exhaust leading to the lower pitch.
As the combustion pressure is reduced, turbine speed falls so impeller speed falls, air entering the turbo falls, balanced is restored between restrictor plate and impeller.

And the cycle starts again

This is neither proven nor researched. Just a quick "theory" idea