welcher, time jitter of D/A conversion basically adds noise in frequency domain. Ethernet data cannot directly introduce jitter, since data delivered is asynchronous to D/A conversion clock and is placed in the buffer. Ethernet cable, as any other cable injects electrical noise - computer/router electrical noise and induced ambient electrical noise. This electrical noise is most likely RF, well above audio band, but product of modulation of such frequencies on any non-linear element might become audible.
Injected noise might also add jitter to internal D/A converter clock. This clock is crystal based accurate and stable, but electrical noise will affect its edges, either internally (before squaring/buffering) or it will add to output edges (that have limited slew rate). Noise added to transition will produce time uncertainty of threshold point - a jitter. Time jitter of digital conversion clock will add sidebands to root signal. Pure 1kHz tone with added 120Hz jitter will produce 880Hz, 1000Hz and 1120Hz frequencies. There will be more sidebands, but much smaller and practically only first two count. They have very small level, well below -60dB (since amplitude of time jitter is very small), but are still very audible not being harmonically related to root frequency. Such jitter will be more audible when correlated to particular noise frequency and less audible when this frequency is low (sidebands are close to root) or with multiple frequencies (uncorrelated jitter).
Ethernet has transformer isolation (or optocouplers) as part of the IEEE 802.3 standard, but there is always small capacitance across, that will conduct high frequency noise (like switching noise).
Injected noise might also add jitter to internal D/A converter clock. This clock is crystal based accurate and stable, but electrical noise will affect its edges, either internally (before squaring/buffering) or it will add to output edges (that have limited slew rate). Noise added to transition will produce time uncertainty of threshold point - a jitter. Time jitter of digital conversion clock will add sidebands to root signal. Pure 1kHz tone with added 120Hz jitter will produce 880Hz, 1000Hz and 1120Hz frequencies. There will be more sidebands, but much smaller and practically only first two count. They have very small level, well below -60dB (since amplitude of time jitter is very small), but are still very audible not being harmonically related to root frequency. Such jitter will be more audible when correlated to particular noise frequency and less audible when this frequency is low (sidebands are close to root) or with multiple frequencies (uncorrelated jitter).
Ethernet has transformer isolation (or optocouplers) as part of the IEEE 802.3 standard, but there is always small capacitance across, that will conduct high frequency noise (like switching noise).