BMW's Z3 is a full-featured, comfortable, powerful car built for performance, speed and everyday drivability. Unfortunately, many Z3 owners complain of a certain seat "rocking" that develops over time. When speeding up or slowing down quickly, the driver and passenger seats may rock back and forth, as if they are not firmly attached to the floor. This is due to the bushings in the seat mounting system, as they wear out over time. Luckily, this problem can be fixed for only a few pennies, literally.
Open the door on the side of the car with the seat that rocks. If both seats rock, simply repeat this penny-fix procedure later for the second seat.
Locate the seat mounts under the seat that are bolted to the floor of the car. There should be one at each corner of the bottom of the seat cushion. Rock the seat back and forth by hand and watch each of the mounts to decide which ones are loose. It may be only one, or all four.
Move the seat in the opposite direction of the worn-out mount. For example, if the worn-out mount is in the front, push the seat backward. If it's in the back, push the seat forward. The seat should lift slightly from the worn-out mount.
Push a penny in the space opened by pushing the seat forward. You may have to force the penny into place. This is a good thing, because it means the seat won't move when you're finished.
Repeat this procedure until each of the loose mounts has a penny wedged in it. Take your Z3 for a test drive. Accelerate and decelerate quickly and pay attention to the seat positioning. It should no longer rock back and forth under normal driving load.