I have had the same problem, but not with a power shower, my shower is fed hot from a combi boiler and cold from the mains.
I contacted Mira Showers help desk and they say it is an imbalance between the hot and cold water at the mixer and installing a pressure reducing valve on the cold side should eliminate the noise.
To test for an imbalance, when the shower makes the whistling noise, turn on a cold water tap at the hand basinor elsewhere, if the noise stops, then there is an imbalance..
Brian Mac, August 2008