Could this be a wheel bearing? I have a siular noise coming from my AP1. I've lived with it for 30k miles now (owned the car since 21k and its now on 56k). Its been through 2 diff & tranny fluid changes. It seems to be when driving at a constant speed or when accelarating in a high gear but mine feels like it is coming from the front of the car. Good luck getting it sorted, I'd like to know the outcome (I guess mines a simular problem to aero3685's)

 

I just swapped wheels back to stock (from 245's on 8"wheels to the stock 225's on the stock wheels), and it seems like my sound went down a little bit.

So it could be really noisy tires... or maybe the wheel bearing and the fact that it doesnt like the larger wheels. Guess I'll just wait and find out.