Engine rattle

[KcajGnowk]

Hi guys, first post but wondering if anyone is able to help as my 58 plate MK5 golf gti has developed an unpleasant engine rattle. It seems to happen when it's warmed up, and doesn't seem to be a pattern to it. There's no engine warning lights, and as far as I can tell it drives fine.  I have taken it to one garage so far and they couldn't identify it.
I have a specialist I've used before for servicing but they want to charge £160(?!) for one hour diagnostic.
I have attached a short video, which hopefully captures some of the sound. Any insight will be truly appreciated.

[JamieSmeatonVW]

Sounds like either a failing duelmass at the worse case or Aux belt tensioner and or Idler pully bearing failing, Any noise when the engine is being switched off?

[KcajGnowk]

There's no unusual sounds when switching off.

The rattle sound doesn't start straight away on on start up, it seems to ease in as it warms up.