I have trouble getting any kind of tone at all when I use the fingering to get C-natural on a D whistle (2nd and 3rd fingers, right?), or when I play the corresponding note on any other-key whistle, so I think it must be something I am doing wrong. The tone I get is both thin and flat. I’ve checked that the holes are completely covered and it still sounds “off.” Is there another fingering for this or something that might help? I have a G whistle, but it is too high-pitched for some songs. Am I going to have to get a Low G in order to avoid the C-natural problem? 
There is the option of half-holing
the c nat–basically covering the top
hole with your index finger slanting
upward so that only half the hole
is covered. Some of the best players
advise students to do it that way.
Here are kind of my rules of thumb for fingering a C natural:
For sustained notes, half-hole it.
For staccato notes, use two fingers. As you say, this one is usually flat unless it is blown with some intensity.
If you’re coming down from a (vented) second-octave D, simply raise your right middle finger.
I hope this helps.
Take care,
BB
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make sure that your whistle uses the 0xx000 fingering, try also 0xxxx0 and 0xxx0x and see if they are better.
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keep at it. I remember thinking there was something wrong with me or the whistle in the beginning. The c-nat will always sound a little weaker and thin if you crossfinger it (at least on the cheaper whistles).
Thank you, my friends! All three options worked much better and give me a nice selection of fingerings.
Unless I missed it, there is also OXXOXX.
Kendra
I have another option for you…send your “almost favorite” d’ whistle to a Pro and have a C-natural thumbhole drilled. The tone will be perfect,not wimpy like a crossfingered c-nat. It will still be a whistle,not a recorder, and you will be able to “pinch” open the thumbhole for “perfect pitch” in the upper octaves. Most whistles play flat in these octaves(modes).
Thomas Hastay.
A C-natural thumb hole sounds intriguing! Where does one find a “pro?”
Landed
There are a great many talented Whistlesmiths right here at C&F. Make a “new post” with a suggested title of…
" Need whistlesmith for mod!"
I’m sure you will get dozens of answers and price quotes.
Kendra, due to a left hand disability, I play a whistle with a C nat thumb hole. I didn’t think of getting the extra hole drilled in an inexpensive whistle, cool idea! My Al D Pro was made with the hole by
Mike Burke. I’d be happy to answer questions about playing this sort of whistle.
Didn’t seen this fingering for Cnat. listed:
(o x o x x x) I find it works very well in the second octave especially on my Sindt D.
Best wishes, Tom