Interesting thread!
Chiff is one of those hard-to-define words ..
Generally speaking it seems most accurate to use “chiff” to describe the chirp at the beginning of a note before it resolves into its resonance.
All flute-style instruments have some of it - it’s part of the physics. Some whistles resolve quickly some less quickly.
There are 2 other sound aspects that tend to get stuck with the “chiff” label:
-
The breath noise.
There is always some of this - it is constant - not associated with note-resolution, but a property of turbulence .. generally speaking, a more efficient whistle will have less of this. Technically it is “white noise” - as the frequency increases, the energy also increases. Many players do not like this noise .. if played alone, it wil be heard, but when one or more instrument is added, the perception of the noise vanishes to the listener. Kavals and Neys have a large amount of breath noise. Personally, I find that more noise in the note allows the player more harmonics to explore using breath pressure, shading, mouth/throat resonance and vibrato. -
Ambient harmonics.
This is a function of the bore width in combination with the window dimensions. One can balance these to produce emphasis on different families of harmonics across the scale .. this produces the signature “character” of the whistle.
For many, the sound of the good old Generation whistle is desirable - its imperfection perfects it in many ways .. a player must submit to the instrument and find the emotional shades the instrument gifts through the loopholes created by the imperfections.
Others have no time for submission and demand absolute control .. for them, an instrument of maximum efficiency and minimum option, nil character and no un-asked-for expression are just the ticket.
Yet other players require some balance of control and exploration .. we are a rainbow 
If you get a mass-produced whistle, it will be what it is - some better, some worse examples of what they are.
If you are dealing with a maker, he/she can customise these aspects to your taste .. however, such customisation will be within the tollerances of the maker. Also, a maker will be reluctant to have examples of their work in circulation that do not represent them.
In this way, the comments above are pretty much on the mark. However, the journey of finding a good whistle for you is still your journey, and yours alone. Just like you, the journey will grow.
Whistles are not a destination - they are a road.
Good luck!