not to sound like a jackass but if it is only in the rear did you put one or both of the rear shock in backwards?

here is what happened to me i kept hearing a poping sound out of my rear passenger side. this was about 2-3 months after my coilover install. i couldn't figure out what it was. then after becoming really frustrated i put the backend of the car up in the air and compared. i had pass side 180 degrees from what it was supposed to be.. turned it no popping..

so try that.