Sound of a zip fastener?

A zip:

• A brief sharp hissing sound.

(AHD)

The sound produced by a slow movement may be just a matter of personal interpretation. Probably a grinding sound:

• A crunching or grinding noise.

I will go with ri[iii...]ri[iii...]

If saying RIRI out loud approximates a zipper-like sound, then yes, it's onomatopoeic. However, I do not hear RIRI in American zippers. Our little plastic-coiled ones for ladies-wear and the big brass ones for 501 Levis and the three-yard long monster-toothed zipper on my camping text all have looong zzzzipzs that the short syllables of RIRI does not seem to represent.

If you are doing a radio ad or TV promo, you don't have to write the RIRI brand. Just record the zipper (going at various speeds) and have a good novelty voice-over say RIIIIIIIIIRIIIIIIIIII, sustaining for the length of the zip, sort of like the insurance co. that has trained us to hear a duck say quack quack but with a distinct "AFLAC" coming through -- the name of the firm. Quora

the word itself is onomatopoeic the noise closing a zipper makes is a zip sound.

Zziiipppp but perhaps only when closed quickly. you could also argue that it is like a growl or i prefer a purr when a zipper is closed slowly.

If you're working on subtitle or caption... the best way to describe the sound since "zip" is a onomatopoeia is mentioning the item first, something like, (jeans zipping)(bag zipping) (pant zips)