fluttering sound on accelerating

What is your question?

I've noticed a fluttering/rattling sound when I accelerate hard (e.g. freeway on-ramp with a slight incline). Once the speed is even, it stops. It's not a knock or clunk or ping sound - the closest thing I can think of to describe it is something like what a thin metal wire in a can would sound like if you shook it, or maybe like the filament on a burnt our lightbulb (again if you shook it). I think it's coming from the front/engine area, but it's hard to tell. The spark plugs are maybe two years old. Could it be a dirty fuel injector?