DSD tracks sound like scratched LPs

Brand new Allo USBidge Sig, Pro-Ject Pre Box S2. Same problem with supplied Volumio 2.79 (I think) and freshly downloaded copy of whatever version of Volumio was current yesterday (3.x) During DSD (Direct) playback, you can hear some clicks happening every few seconds, like a scratched LP. The noise is also accompanied by what I can only describe as a momentary slowing of the music. Files are on NAS, and I just happened to notice that the noise seemed to coincide with the activity led on the NAS. The DSD buffer was set to the default value, so I increased it, but no change in sound (the led did flicker less often, though). Oh, this is on Ethernet, no wifi.

Probably unrelated, but I also seem to experience the hissing sound when first playing a DSD after a FLAC, which has been reported elsewhere with this DAC, and can hopefully be addressed eventually.

Thank in advance.