Creaking sound when pedaling [audio clip included]
I found the culprit(most likely): it was the cogs digging into the freehub body, as seen in the post here. I imagine the cassette lockring wasn't tight enough. The recommended torque is 50Nm but since I don't have a torque wrench that big, I've tightened it how tight I believed was enough, last time I removed the cassette. 
At the time of removing the cassette, I've also greased the derailleur hanger bolt and contact surface, and the skewer, but I believe the noise came from the cassette.
Sorry for wasting your time !