From what you are describing it does sound like a bearing that is starting to fail OR most likely a poor quality bearing. I'm wondering if the difference between the pool store test and yours at home is the mount. At home is it mounted to something that could act as a sounding board and in a way make the noise more noticeable? In any case since you are unhappy with it they should make good on the warranty. Should be nothing out of their pocket. Keep complaining if they keep balking. If you get as loud as that motor they will eventually pay attention. Sorry I couldn't be any more help.
Al