It could be a couple things, the pump may have air trapped inside, it may have a loose bush bearing, (this is a collar that wraps around the hole the drive unit inserts in) or it may have a cracked drive unit or a defect in the motor. We had some motors recently that the plastic inside the well the drive unit is inserted in has a ridge. This was a defect in the batch of plastic. Check for these things, let me know and we will send you what you need.