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@aldofern, I've had the HDMI "snow" issue since late 2012 Mac Mini -- and learned to just live with it. The anomaly has no pattern, is intermittent, sometimes doesn't occur for hours. I used to think it was triggered by "display sleep" mode but since Mojave it happens any time and some days several times in the space of minutes. Turning the monitor off then on always fixes the issue until it reoccurs.
Periodically I check to see if someone's found a solution. 7 years is a pretty good innings and I'm ready to get a new Mac Mini but not if the problem still exists - 'cause I'd love to run 3-monitors!
I need to run two monitors. The 2012 Mac Mini has only 2 display ports: Thunderbolt and HDMI. Like others I've tried different monitors and all the various cable connect options including new HDMI cables. My current setup is HDMI out to DVI adaptor into monitor (my favourite monitors do not have HDMI input).
Reading through the various responses here I see a repetition of
failed solutions — hopefully someone can confirm if they've actually resolved the issue? For example, the suggestion to enable "power nap" is new to me, so I'm trying that now... and so far OK.
AFAIK the issue is only related to HDMI. I had read somewhere that the "snow" problem occurs because MacOS has some kind of incompatibility with HDMI
refresh rates. The following link discusses this (I think — a bit above my tech knowledge though)
https://discussions.apple.com/thread/6500141