It could also be a network issue/misconfiguration.
But it is hard to tell without knowing details of the network. Are there other network devices, that are not listed in your initial post, like powerline adapters or managed switches?
If you have an usb audio device, you could try connecting it to your core to check if this issue persists when playing audio locally without any network involved.
Is there any special configuration on your diskstation (like jumbo frames enabled, link aggregation…)?