Thanks! In the meantime, I found the solution, but unfortunately I have no idea why this fix the issue. By specifying the sink device as follows, the noise disappears:
gst-play-1.0 --audiosink=‘alsasink device=hw:CARD=1’ ./file.dsf
and in mopidy.conf:
[audio]
mixer = software
mixer_volume =
output = alsasink device=hw:CARD=1
buffer_time =
No other soundcard, DAC or any other alsa device/card in the system, so when the problem was present, it was the only device gstreamer could use. Strange…