Rattling Engine at Shut down

My engine makes this ratting noise that sounds like a metallic object left flapping around inside when I turn the engine off.
The background story is that my hybrid system on my 2013 Prius is down ... again after a fail HV battery rebuild project. So at this time the HV battery is not charging or discharging and the whole car is running on the gasoline engine only. my obd2 code are P0A80 and P3024.
P0A80 is a familiar code that I get when battery cells fail, but this P3024 started showing up at the same time as the rattling issue.
I know I have to address the HV battery issues, but I don't know if the rattling issue is related to hybrid system not functioning or if it a completely different issue. Are the motor generators MG1 and MG2 responsible for bringing the gasoline engine to a quite and smooth stop?
my second question is whether it is safe to drive it on gasoline engine only.

 

Thanks for replying,
It has 160k.
I read quite a bit about that egr circuit issue, and given my mileage it's quite possible. I'm still a little hesitant on that route because my symptoms are not quite the same as others. Most people seem to experience rattling when slowing down while driving or while idling. In theses cases a constricted egr circuit seems to be preventing the pressure threshold in the intake manifold to be made.
Mine only rattles when I push the button to shut the car off, it feels like something is spinning freely before coming to a rattling stop.
Unless I'm mistaking my understanding of the egr system, it should have no role during the shutdown process.
My engine rpm gets up and my idle is very laud though, I've seen a couple people mention the same issue, but not clear if it is related to the egr or the HV battery issue.

 

Thanks for replying,

Well it is starting and driving, except it's not charging or discharging the HV battery. I am just not sure how far it can go in this condition.