I recently stopped listening to the radio and play my own music playlists. I'll typically be a few minutes in to my drive before I start up the playlist. When I started doing this, I noticed the metal-like clunking as well and thought something may be wrong. Never heard this on any other vehicle before. I stopped by the dealership for a regular service I was booked in for and mentioned it. The service tech was immediately able to, without any reasonable doubt, assure me that it was, as mentioned, the ABS self check. That was a nice relief as the sound is rather ominous.