I have two of these and when one of the two goes to 40% (or anything other than 100% for that matter) it vibrates/buzzes loudly. The tank is in wall- accessed by a door in the next room and it still can be heard through the tank..
You either have a loose bearing or a damaged bearing. Try removing the bearing bush and upper bearing, check them for any calcium build up or damage and reinsert them being sure the bearing bush is in all the way. These are parts 3000.62 and 3000.61 respectively.
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