Rattle/ Vibration from engine bay during idling

My 2010 Malibu LS 2.4l 4spd developed a weird rattle vibration a few months ago. It isn't the engine knocking, there is no CEL or code when I read the OBD. The car is performing like it always has. The rattle is like a metallic vibration sound that only happens when the car is idling and stopped, never when its moving or throttled over its idle rpm. I looked under the hood, the belt seems fine, no cracks, the A/c compressor is kicking on like normal. The sound doesn't stop or start with A/C on or off or blower fan motor. IT doesn't seem like an interior rattle. My only guess would be a loose belt with bad teeth or maybe engine mounts, but either seem unlikely with 90,000, never had a GM car with a bad engine mount. The only thing wrong that could be related would be a few times during this hot summer 115+ degree heat and during stop and go traffic, the A/C compressor would shut off and blow hot, but the car never showed anything above half way on the temp gauge. And after moving again, and turning the a/c back on would blow cold like normal. Also, about 5,000 miles ago I replaced both Oxygen Sensors front and back with gm ones since CEL was on. So, maybe A/C clutch is failing or maybe belt is out of whack or motor mount?

Any ideas or help would be awesome, i'm about to go crazy with this rattle noise while driving...