Why Do I Get This Crackling Noise on Zeroing out the High Frequencies? I recently started playing with the Fourier transform (after spending a few weeks learning about the mathematics behind it). I decided to try hacking together a low-pass filter on the following sound bite: 


            
            

In particular, I took the Fourier transform, zeroed out the highest 1/2 of the frequencies, and then took the inverse Fourier transform. This is what I got


            
            

Why is there that crackling noise?
Two potential issues:


Doing filtering in the frequency domain (using an FFT) requires overlap-add, overlap-save, or a related algorithm. This is caused by the difference between linear and circular convolution. Otherwise you get time-domain aliasing
Some of the noise sounds like simple clipping. Filtering can actually increase the time domain amplitude for some samples and if that exceeds the available range it either clips or wraps around.