My car just started making this sound as well. Only when cold and only lasts a couple of seconds (almost sounds like a heat shield vibrating but most certainly is coming from the area directly behind the cam adjuster magnet unit).

I've been told by my local wrench it is the Cam Adjuster Solenoid failing. I've disconnected the cam adjuster magnet and the noise stops (probably because the magnets actuate the solenoid, right?).

I'm wondering if anyone else has had this noise, ONLY replaced the magnet unit and been able to solve the problem? (The magnet is <$50 versus the solenoid which is closer to $1,000). Seems like even if it doesn't work, it might be worth TRYING the $50 solution first anyway.

Thanks for the help