ISA 430 (Mk1) 'ess' crackle?

[christianmurphy]

I picked up a used isa 430 producer pack (mk1) today and it's great. Currently running that with onboard digi card (isa generating clock). Wanted a decent all in one solution for kicking my vocals up a notch. Running via spdif (coax) to pro fire 2626 digi input.

All sounds great other than when I was having a fiddle with the de-esser to see what it's like. Noticed when I made 'ess' sound it made the system crackle somehow (sounds like vinyl kinda noise). I didn't notice during normal tests whatsoever. Any idea where it's coming from? (Have yet to check analogue out without DIGI yet).

Audio of recording of it:
https://soundcloud.com/murphkid/isa-430-ess-clicking