2019 CX-5 Turbo Engine noise/rattle

[CX5aus2.5T]

Hi all,
I have a 2019 CX5 GT 2.5 turbo (I'm in Australia just for reference)
For about 18months I've had a rattling noise coming from the engine bay, sounds similar to pre-ignition/pinging. I always use premium fuel (RON98). It most frequently occurs at highway speeds (105-120 km/h, 2000-2200rpm), when cruising up a very slight incline, or accelerating slowly from about 105 up to 115. Most commonly occurs on warm days (above 25c) and once the car is at operating temp.
I've had it at the local Mazda dealer 4 times trying to diagnose it with no luck. ( was still under warranty until recently) They've checked everything from exhaust & heat shields to eng and trans mounts. Windscreen cowl and even bonnet protector, they've come up with nothing.
Has anyone come across this, and found a remedy. Or have a suggestion on what else to look at.
Thanks for reading my long-ass story

 

[HyFlyer]

Have you ever changed your spark plugs? If so, OEM or aftermarket?

 

[CX5aus2.5T]

Have you ever changed your spark plugs? If so, OEM or aftermarket?

Spark plugs are still original. It only has 48,000km. Mazdas service schedule lists plugs at 60,000. Perhaps I'll look into that, replacing them early can't hurt.

 

[cz5gt]

Assuming its nothing from the engine itself, spark plugs/coils (i. detonations) or loose heatshield or something...
in US there is a tsb for rattle coming behind the shield for the heater core pipes and bracket in engine bay. but its more pronounced at idle. Have you checked that nevertheless?