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According to the background of the early legendarium, Noldorin derived from the Old Noldorin, the language spoken by the Gnomes/Noldoli in Valinor. That language was quite similar to Qenya, the language of the Lindar but had some differences.
A celtic-sounding language spoken by the Gnomes/Noldoli existed since the beginning of Tolkien's mythology. In the first stage, it was called Gnomish quite similar to later Noldorin/Sindarin but also quite different.
Noldorin with all its backstory existed even while Tolkien was writing LotR. The Elvish phrases and languages, while now called Sindarin, were 'Noldorin' in Tolkien's mind while writing them. It was only while writing the Appendices when he decided to retcon all the backstory and give "Elvish" a new history and name, thus we have Sindarin.
The Etymologies Noldorin is so similar and compatible to Sindarin, that Tolkien consulted it while working on Sindarin, by "updating" in his mind the old words with the new data. This methodology is used even now by Elvish students when they need to "import" or "update" Noldorin words into Sindarin proper.
Example of differences
For example Old Noldorin initial l-/r- became lh-/rh- in Noldorin respectively, however they remain the same in Sindarin. Also, the Old Noldorin diphthong ai becomes oe in Noldorin but ae in Sindarin.
For example according to the Etymologies, the Old Noldorin word raika evolved into Noldorin rhoeg but in Sindarin, the very same word should be "updated" into raeg.