Sorry for the poorly worded title. I came across this excerpt while reading, and I really feel like it should be gramatically incorrect. The sentence structure in the title fits for the part before the comma, but not after. However, I plugged the part after the comma into some grammar checkers and they were all fine with it, implying "How pleasant the noises they make" works as an independent sentence. How does it work? Isn't it missing a noun because "noises they make" is the subject? I would expect the "are" in brackets to be there, but it wasn't in the original text.
How detestable I find those insects, how pleasant the noises they make (are) when I crush them. It is strange, truly; I despise the silence of their absence more.
Thanks in advance everyone!