Sorry, I wasn't suggesting that you could eliminate the whistle type noise in the first iPurifier3. That unit was defective and returning it was a good call. I was talking about digital type noise that presents as jitter, etc in your DAC. Nothing that you would hear as an actual sound, but as deteriorated SQ.
I find it odd that you can't hear any difference with the iP3. I have the iP2, which had a net negative effect used in between my sms-200 streamer and the Korg DAC. Used with my Dell 7000 laptop, it improves the sound. In either case though, I can clearly hear the difference when the iPurifier is in the system.