I've got four of them and have used them extensively since they came out and never had one whistle. The only thing I can think of is that all components are not screwed in enough. Are you aware that the drip tip actually unscrews from the metal top which again unscrews from the tank? If it's not tight that could cause a whistle. Or if the whole unit itself is not screwed in properly to the PV or even if you've not unscrewed the drip tip and cleaned it properly? Just a few ideas but it does sound a bit strange.