Rattling sound on Gas / Engine

I have Prius Alpha (Prius V or C) 2013. I hear a rattling sound coming from engine / transmission area. The sound only comes driving on petrol / gas while the car is on low RPM. On EV mode, there is no sound. I got the spark plugs checked, air filter changed, fuel filter, air filter, throttle body, injectors and EGR cleaned but the sound is still there. The transmission oil has 30k kms on it and is still maroon with not metallic particles (as per the mechanic).

What could it be?

 

Even got the catalytic converter cleaned.