I tried to add noise with randn function but its not filtered by butter filter. How to add high frequency noise in a .wav audio so that I can filter it with butterworth filter?

%Butterworth filter design

 n=10;
wn=0.4;
[a b]=butter(n,wn,'low');

%Reading Audio Signal

 [Data,fs,nbits] = wavread('sound');

 Data=Data(:,1)';

 NoiseData=Data+.005*randn(size(Data));

 wavplay(NoiseData,fs)

 DataOut=filter(a,b,NoiseData);