Never locked my doors before closing them, but I did have some strange symptoms something like yours. For example, I could not lock the Driver door from the passenger side. Maybe the popping noise is a lube or alignment problem in the passenger door locking system?
I have not truly mastered the Germanic intricacies of the locking system, but seems to flow naturally from using the passenger door only with a lady passenger, locking and unlocking doors for her with the master key, and keeping valets out of the trunk and glove box by passing them the other key.
I found a surprising solution to my particular symptoms.
I bought a new master key from the dealer for my VIN# just to have a spare. The New master worked all locks just as the manual indicates locks should work. Not so with the old, worn master. The master and secondary key do not work all the locks the same way.
I have no idea if you problem is a worn master key, but if all else fails....
They that can give up essential liberty to obtain a little temporary safety deserve neither liberty nor safety. Benjamin Franklin, A.D. 1759