From off to on, this can be normal, but it can be reduced, if it is from low speed to high, that is not normal. When the pump comes on from off, the drive unit gets pulled down and slaps the bottom of the pump, the 3005.740 is a shim to minimize this travel with a rubber shock absorber to reduce the noise. On the newest pumps we use a thicker piece 9011.740, you can use this piece or a second 3005.740, but only the plastic disk stacked on top.