I've had this rattle for a while. Placing something between the gas pedal and its mounting bracket did help reduce it a little bit, but what reduced it the most was removing my 18" wheels with harsh tires (PSS's) and putting the stock wheels / tires back on. The stock tires seem to filter the kind of vibration that causes this much better.

My car is supercharged with straight pipes coming from my catless header all the way to exhaust tips. Everything rattles in my car LOL. I just stopped givig a shit LOL