Matlab - Average White Noise PSD

Hey, I am currenctly facing the following problem: I have an audiosignal-vector y with a length of about 1e6. I want to add white noise to this signal and display the noises psd, which should be as flat as possible. The way I tried it:

[y,fs]=audioread('test.wav');
snr=5;
r=awgn(y,snr,'measured')-y; % There are other ways to create wgn but this shouldnt be the problem

MATp = zeros(length(r),50);   

for i = 1:50
    e=awgn(y,snr,'measured')-y;
    [q,w]=pwelch(e,[],[],length(e)*2-1,fs);
    MATp(:,i)=q;

end

p=mean(MATp')';
plot(w,p);

This works okay, but the psd is still far from flat. With 10000 psds to average it should be fine but this is way uses way too much memory. I found some other solutions on the internet but they all just worked with very short vectors (<<1e6). Are there any solutions for my Problem?

Thanks! Klaus