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.
Saturday, October 27th, 2012 AT 8:00 PM