Heat shield fell off? I had one fall off my van and the front header slowly melted my fan. I would think it's gotta be something outside the engine, under the hood. If it was IN the engine, I don't think you would smell it when you were moving. It would come out the tailpipes. Unless you have a breather in the VC.

Anyways, I'd be looking all around the headers for anything out of place/burnt. Or like Bill would say, just turn up the radio and drown out the smell