I would start by checking the heat shield. I HOPE that is the problem. The slow oil leak concerns me especially with this engine. There has been many reports of timing chain issues due to extended periods of low oil. This generation of V6 engines had issues with burning oil.
The best thing you can do is try to isolate where the rattle is coming from. If it is the heat shield, the noise will come from under the vehicle rather than the engine bay.