Not familiar withe the Symphony, but maybe remove from the chain to eliminate it.
Saying this, I experienced similar when initially connected my Iris to my old Ares 12th-1; I thought I'd hit on the correct configuration (010, 000, 100 etc) but I hadn't. Even though I was getting music, I was experiencing the same static type pops and crackles, tried configuring again via the Ares front panel buttons and finally found the correct pin combo.
I now have the Pontus 15th connected to the same Iris, and had no such issues and didn't need to change the configuration
Hope you solve this 👍