If you’re experiencing a crunchy or noisy shifter on your NuVinci hub, this is generally due to an issue with the shift cables themselves. We recommend replacing the cables to ensure prolonged proper operation. Take a look at the Park Tool Repair blog that offers instructions on how to replace these cables: https://www.parktool.com/blog/repair-help/nuvinci-n360-hub-installation-and-adjustment
Also - here’s a video from Enviolo that highlights the procedure: https://support.enviolo.com/hc/en-us/articles/360003208591-Cable-Installation-of-Nfinity-
The shift cables are standard bicycle derailleur cable, available online or any local bicycle shop.