Most probable is the drive unit just fits tight on the shaft, as a result instead of easing down it skips and smacks the bottom creating a click. This often resolves itself after a couple weeks of wearing in. I would give it two weeks, if it persists, try swapping the drive units between pumps, if it follows the drive unit, we know that is it and can send a replacement. If it does not follow the drive unit, the part 6055.740 is the likely culprit, this is a shock absorber/shim under the drive unit seated on the shaft, in some cases a second shim is needed or possibly this part is missing but that is only likely if the pump was disassembled, they can fall out.