All of my delayed rattles have been exhaust related. My Jeep, motorhome and diesel pickup. Inspect very closely, there is metal touching metal somewhere. My GMC it was the bracket that holds the exhaust as it leaves the manifold, my jeep a crossover bracket connected to the cat and my "C" a exhaust shield bracket under the shield and difficult to see.
I found my rattle on my C by driving home after smog inspection with the dog house off, kind of tough to do and only a few miles but the rattle noise with and without the dog house sounds like two totally different areas.
To check your motor mount, raise hood, apply parking brake and gently accelerate and see if the engine tilts or lifts. You can have someone watch from a safe distance or record with your phone. Hold foot brake as well, just a touch on the accelerator will do it.
