I have sometimes too some crackles from rear deck on small holes, that's where you see that the M5's suspension is sportive oriented. Make sure you do not have anything in the free aera where you have your "auto-repair kit" for tyres.
I noticed that the cable of the BMW-phone do a clear noise (hitting the rear console) when the car hangs on hard accelerations.
I had a very irritating crackling sound coming around me, first i thought it was from under the seat, but un fact it was the leather of my seat that was too "dry" (washed some times at it is the silverstone nappa getting darker) and always making little noise when the up-part and the down-part of my seat had a contact.
I used some BMW leather-care product and no more crackling at all ! Do not forget to use such product after cleaning your seats ! Water with soap is enough to get leather crackling !