Thank you so much @Chancee, @EightiesDuntonFordie and @jimbobford36 for the solution + clear images!
I've been driving a left-hand drive Fiesta ST-Line (latest model 2019) for one and a half week now. While I am loving it so far, this buzzing noise under load/throttle around 1900-2000RPM was becoming very annoying. The noise being on the left, so the driver's side, made the buzz very obvious even while having music on at normal volume.
It took quite a bit of searching online before I found your solution, and I feared the dealer would probably not know (or care enough) to solve this. Luckily it turned out all that was needed is the temporary removal of the air intake duct and placement of 2 simple tie-wraps!