Well, I can't say there is really a confirmed answer to this, but I can say this:
My Illogical Theory
I believe that it may be the common hissing sounds that snakes normally make in real life, and you may be mistaking it for 'ch' instead of 's'.
I think this is true. This is something that I think a cobra would do. And this is probably the best time for the snake to do it - it's name begins with a vowel so it's a good way for Game Freak to add the 'hiss' sound.
I haven't watched the anime in a long time, so it may be quite a clear 'ch' sound. But this is probably a reason. :3
Hope I helped! ;)
Sources: My wonderful illogical theories. xD