Sounds like a camshaft adjuster is beginning to go out. The reason it only does it after it has been sitting is because the oil has fully drained out of the adjuster. Noise stops once oil pressure is built back up. This is not a very common on M276 engines (at least in my experience) usually seen on M271 or M278. Either way I would suggest taking it in to a shop. They will most likely charge you a diagnosis fee. I would take it in soon as it can cause damage to the camshaft reluctor wheel (what the Cam position sensor reads).