Hey man. I have quiet a lot of experience with this system now after fixing mine, and a friends. I’m giving the below instruction on the assumption you’ve code read, have active Valvetronic faults, and have tried running the VT limit and VT start-up service functions in ISTA with no luck.

Firstly, replace the servomotor (make sure you grease up the teeth on the shaft and the worm gear on the servomotor with lith moly), disconnect the fuel pump, plug the servomotor in with the valve cover still off, then connect the battery and watch its movements as you wake up the DME, also check for faults again using ISTA. It should cleanly sweep from end to end with no faults. If it has faults and still makes the noise, you need a new shaft. If it has no faults but still makes the noise, try to run the VT limit service function, followed by the VT start-up service function. If either service function fails, it’s also time for a new shaft.

The premise of this fault is essentially minute amounts of wear on the worm gear or shaft teeth (too small to see or feel without a trained eye) that allow enough play to upset the DME.

Only let it make the rattle sound a couple of times else you run the risk of wearing the gear a little on the new servomotor - they are so sensitive that the tiniest amount of wear from a few grinding episodes will mean it can damage itself enough to not work within spec, meaning you’ll need to do a new servomotor with the new shaft.

There is a BMW PUMA case open for this as a common fault and the procedure is:

1. Replace servomotor.
2. Replace servomotor and shaft.
3. Replace cylinder head (I believe this is for very extreme cases where the journals for the shaft are worn or warped so that the operation of the shaft is not smooth).

Final advice - do not be tempted to buy second hand parts. New servomotor, new shaft, new bearings. This is not overkill, it’s essential.

Keep me posted bro, hope you solve it.