A couple of my cars including my audi do a pop/crackle on downshift with high revs but only when i hit the accelerator while its revving down if that makes sense? so i shift up to 4th or 5th, then down shift through to 2nd and revs are high as im slowing down, i give the accelerator a tap and it pops and crackles. only happens on a larger high flowing exhaust though, tried on a car with stock exhaust and nothing.