Here's an interesting noise-based Love2D shader. Simply keep setting factor to a random number every frame (or some other interval).

gistfile1.c

extern float factor = 1;
extern float percentage = 0.15;

float rand(vec2 n)
{
  return 0.5 + 0.5 * fract(sin(dot(n.xy, vec2(12.9898, 78.233))) * 43758.5453);
}

vec4 effect(vec4 color, Image tex, vec2 tc, vec2 sc)
{
  float grey = 1 * rand(tc * factor);
  vec4 noise = vec4(grey, grey, grey, 1);
  return (Texel(tex, tc) * noise * (1 - percentage) + noise * percentage) * color;
}