Static issues on Interface+Monitors system

Hello,

I have inherited a sound system – two monitors (Mackie HR824mkI) and a Motu 828mkII.
The previous owner ran the system off Firewire but I will not be able to do the same so have been trying to find another way, only interested in getting music output for now. After a couple of hours of tinkering I’ve connected my PC over optical/Toslink and managed to route a sound signal to the main outs, so I’ve got sound! However I have some problems as follows:

1. There is a very(very) slight static during playback
2. After sound has stopped(music ends, paused, etc.) there is quite heavy static that plays for around 5 seconds and then stops.

During troubleshooting I noticed that if I play music that is stored locally(not streaming) problem 2 is solved, although problem 1 is still present. My PC is plugged in over LAN, would there perhaps be interference here?

After some research I’ve tried(to the best of my understanding) to prevent a ground loop by running all the equipment off the same socket. If it’s relevant it's also plugged in to an 'ELC black box' which is how it was previously set up.

Sadly the previous owner passed away, and therefore I don't have much help during setup, which is why I came here. Please guide me if I have not posted this in the correct sub or have not given enough relevant info.

Thank you!