How to Concatenate white noise signal and applying bandpass filter matlab
If I understand the question correctly this should solve your problem:

L = 5000;
P = 10000;
sigma = 2;
mu = 0;

left = sigma*randn(L,1) + mu;
right = sigma*randn(L,1) + mu;

y = [left zeros(L,1); zeros(P,2); zeros(L,1) right];    % stereo sound matrix