I've had a good listen through the trusty AKG K550's and it sounds very familiar. My original silicone filled mounts are shot and the engine shudders badly when garaging but not whilst driving or first thing in the morning. The mounts must soften as they heat up and allow freer movement which then amplifies any existing noises as they are too solid to absorb vibration effectively......
Which leads me on to the possible cause; If it's anything like mine, you may have a slight turbo heat shield vibration. My car makes this noise and the shield rattles at idle because of the bad mounts. It also sounds like a bearing howl but not in a rough, scratchy way though. The timing belt idler pulley bearing is a good suspect as you say, as could be the timing belt covers.
I really think it's a turbo shield or other cover or thin metal part, maybe even an induction howl. Try squeezing the various induction hoses and also operating the throttle from under the bonnet to find the harmonic sweet spot, (if there is one), further up the rev' range.
Can you do a video with the camera right in close to the various nooks and crannies with a tiny wee bit of throttle?
A mechanic's stethoscope might also pinpoint it.
After another listen and realising, (I think?), that it wasn't your video, it does get much louder when you/he/they lean/s toward the turbos and iirc mine sounds like an airflow related issue through that area that I think peaks and dies off as I increase throttle too.
I think!