What tyres are you using and are they pretty new?
Any chance it is the wheels rubbing? (either front or back)
The rears would rub the soft padding first and make sort of a low caaaawwhhhh noise.
I remember a good while back you mentioned some rear padding issues when your car was new.
I experienced this once once with heavily loaded car on an uneven surface, but my rear dampers were set to a very soft setting (zero in fact!).
The front wheel arch lips rubbing the tyre sidewalls makes a more
kkkkeeeeehhhhh like noise.
I had this problem recenty with a 225/40/18 8x18 alloy tyre (ET 38) combination and had to get the insides of the arches rolled which fixed the problem.
However while I can't swear to it, now that I know what front wheels rubbing defintely sounds like I can't rule out that the standard wheels rubbed slightly under extreme track use with new tyres and high pressures (plus much stiffer springs than normal, but same ride height).
Maybe check the paintwork under the arch, although it suprisingly doesn't actually get messed up too much from rubbing - the tyres get much the worse impact.
Perhaps put some bluetack or something there and see if it gets smeared?
One other idea, is your sump guard bedraggled and hanging down a little?
I haven't removed the rear dust guards (heat shield?) myself.
I would guess that the high speed nature of Donnington won't present too much a of a challange for the rear discs.
Also I suspect that the increased braking force of your car due to DS2500's would mean that the rear will become lighter due to increased weight transfer under max braking and the rears will actually be doing less total work than if the car had standard front brakes.
Just a theory, I can't swear that its fact
Cheers,
TB