I'm too rusty on matlab to help directly, but here is how to generate good quality white noise, or indeed, any other shaped spectrum.
Create a 2^n long array for the modulus, and shape this to the desired profile against frequency
Create an array for phase, and fill it with random numbers between 0 and 2 pi
Do an IFFT of this data, to give a real time series. This is a time history of statistically valid Gaussian white noise.
Cheers
Greg Locock