Thanks for the reminder - video with sound when starting cold can be accessed here:
In addition to the normal engine sound when starting cold you should hear this high frequency sound like a slipping belt
Tried to identify its origin and it seems to be loudest at the left side and back end of the engine block / starting point of the cardan shaft (not sure if you hear this on the video)
Pulling the clutch doesn't make a change and the sound disappears after 3-5 minutes when engine is warm (whereby you hear it consistently until engine is warm)