Had the same symptoms on my 330d E90 LCI.
Slight variations at idle on first morning startup. If you press the acelerator a couple of times and bring the RPMs above 1000, problem would not return until next morning startup.
It was a problem with the swirl flaps and it cost me over £600 - BMW only sells the whole intake set.
I wonder why they come up with one diesel engine after another and they all seem to have this problem...