My wife and I just purchased a 1998 Plymouth Voyager from a car lot, and noticed a rattling/vibration coming from the accelerator pedal. There is a very mellow rattling sound coming from the passenger side of the engine. The rattling/vibration can be felt through the pedal during acceleration only. The rattling can be heard clearly with the hood open. I've checked all the fluid levels (oil, transmission, brake, power steering) and the P/S fluid seems to be low, though I don't believe that to cause an issue with a gas pedal. The transmission fluid looked a bit low yesterday, so I added a quart of fluid but no change to the problem.
Did you use ATF-3 transmission fluid? The wrong type will cause a shudder during gear changes and torque converter lockup.
For a noise you can hear but not pinpoint, use a stethoscope to poke around with. Harbor Freight Tools has them for less than five bucks. Auto parts stores have them too.
For noises that only occur while driving there is a tool you might be able to borrow or rent from an auto parts store that borrows them called the "Chassis Ear". It is a set of six microphones, a switch box, and headphones. You clip the microphones to suspect points, then drive around while listening with the headphones. You can move the microphones around to zero in on the source of the noise. Be aware that many mechanics have never seen or even heard of this tool. Suspension and alignment mechanics use it to find rattles, squeaks, and other noises.
Worn inner anti-sway bar bushings are pretty common and real easy to replace, but I don't think I'd call that a rattle. It's more of a light thumping sound that goes away when turning slightly.
Since it's felt in the gas pedal, look for something rubbing against the throttle cable or the body. Check the exhaust system hangers. They always have some type of rubber isolator to separate the two metal parts. If those metal parts touch they will transmit normal engine vibration into the passenger compartment.