How to prevent "zipping" effect on a modulated fractional audio delay line? (Flanger)
It's a bug.

M_PI*config->count++

You are processing 2 channels, but incrementing your modulation in the inner loop. So at the end of processing 1 channel, you continue to increment the modulation as you process the second channel.

c1 [ 1,2,3,4]         [9,10,11,12]...
c2           [5,6,7,8]


The modulation has a discontinuity, i.e in my simplifies example - c1 modulation jumps from '4' to '9'. resulting in zipper noise. Hope that makes sense.