As the 40kHz is well beyond human hearing, I thought that the unit would operate silently. However, the metal cage buzzes against the bowl, as does the lid ( which is most ineffective at keeping the heat in ). Some duct tape between metal parts provides sound insulation, but any metal objects placed inside the cage for cleaning still buzz. It's not so loud, but its a really ugly high pitched sound that drives me crazy. So I just leave it to run in a separate room and it's no issue, but I was hoping it would sit on my workbench to be at arms reach.Strangely, my dog pays no heed to it at all.