My delightfully loud Syn High D has one little irritation: some finger changes produce a squeak on the side, specifically middle D to C. The Syn needs four fingers to produce a good C, i.e. OXX XXO.
Is this an inevitable fault of the whistle design, or can I fix it with better technique?
This is typical of recorders and whistles (I don’t know about other woodwind instruments) - slurring from one register to another causes a brief clicking noise. Just which notes it occurs between depends on the instrument - I’ve found some that do it when going from B to E in the second octave (D whistle), for instance. When playing recorder one generally tongues the second note gently (a soft dh or even y tonguing) if the register change happens in a slur. On the whistle, you could insert a cut or something.
It could be a matter of sloppy fingering, but if you’re fingering a four finger C, then you’re only lifting one finger to play D-C, so that seems unlikely.
Since I don’t really slur that much on whistle anyway, it isn’t such a big problem for me. But it’s a feature of the instrument, not so much technique (although technique can be used to mask it).
I’m wondering if your “squeak” is actually a second-octave E “trying to get out”. My Clarkes especially are very happy with OXX XXO as a reasonable approximation to an E, useful from a OXX XXX D but I can see it being a nuisance if I was hoping for a C (which does happen if I blow very much softer). Perhaps slurring may not be an option for these two notes on your whistle?
An oxymoron … all high D’s are irritations 
j/k
Yes you’re probably producing some in-between note, like a C# somewhere in the finger changes, or a flipped note as others suggest. Those ‘dirty’ notes in between the music, often give it character. I must say, I like them, except in classical music, when added notes are most unwelcome!
Is this an inevitable fault of the whistle design, or can I fix it with better technique?[/quote]
When I’ve found that happening, I’ve mostly found it to be imperfect technique on my part. Once in a while it will be a dud whistle, but since I can tinker with my own home-mades, that gets fixed or recycled. It should be easy to tell. If you can play the change slowly without the squeak, then it’s a fingering issue.
S…l…o…w…l…y. Duh! Thanks.