Brought it into shop, they said it was a bad bearing on the alternator. Got it replaced, and it's still making same noise. Does anyone have any idea what it could be? It's most prominent when the engine is cold and goes away under light acceleration but then comes back. It does it randomly when the engine is warmed up. Doesn't do it really when the car is in park, only when it's in drive. Sorry i know its confusing, it's been tough to reproduce. Sounds like a rumbling along with the engine.

 

here's another video

 

possible it could be the front flanges on the Y pipe leaking?

 

The dealership may be able to figure it out better than a local shop. I know they'll charge more, but may be the best way to get it figured out.