Still no update on this? I have a very similar problem: Audio noise when input channels enabled, despite only playing a simple sine wave or something…
Using PulseAudio for both input/output, Xonar D1, Ubuntu 18.04.
Noise goes away if I reduce buffer size to 128 (default sample rate 44100Hz) or increase sample rate to 176400Hz (default buffer size 512) but anything in between get a pulsing buzz sound added to it