Just bought my first whistle, a Generation “Folk” D, and of course had to “tweak” it. Strangely enough it worked. It originally had a bit of a hiss and after performing all of Dale’s suggested operations the sound is much improved.
I then decided to go one step further and make it tunable up as well as down. Using a plumber’s pipe cutter (used for cutting copper water pipe and available inexpensively at any hardware outlet) I removed about 3/16" from the mouthpiece end of the tube. This must be done very slowly and gently so as not to crush the tube, but the result is a whistle tunable in both directions. A good idea is to replace the mouthpiece temporarily before cutting and mark around the tube at the point where it is set by the manufacturer. This is a good starting point when tuning to other instruments.
Dave