20.04 – HDMI input switching causeing popping / crackling with USB Audio – Pulseaudio problem?

I seem to be having issues with HDMI effectively sending noise back to my PC when disconnecting or changing inputs on my HDMI switch.

I don’t use HDMI for audio and have disabled any HDMI audio in pulseaudio.

I have limited space on my desk and my monitor only has 1 HDMI port so I just want to use that and have ben using a 5 way HDMI switch for years. However, recently, whenever i change from one input back to my PC, a load of crackling and popping randomly on the right or left channel seems to occur during playback of any audio for quite some time. If I don’t change inputs again on the HDMI switch for 10 minutes or so, this noise can eventually go.

For my audio, I use a USB DAC and as it is digital, I don’t see why I should be getting this interference. Even when switching HDMI inputs, i get a brief half second pause of my computor audio, and that seems to be when it gets messed up. The strange thing is that if I use the headphone/speaker output from the motherboard or front of my PC, the HDMI switch does not introduce this crackling and popping, so why would the digital output such as USB do this? That in my opinion should be far cleaner!

Another way I tested this to see if it wasn’t my HDMI switch at fault was to use an HDMI right from my PC to my monitor. Unplugging it from my monitor doesn’t do anything to the audio, but plugging it back in instantly introduces all this noise in the same way as switching back to my pc input on my HDMI switch. If I use DVI from my graphics card to my monitor and repeat this process, the noise is never a problem.
I then thought I could maybe get a DVI to HDMI cable to prevent audio going to the HDMI switch or bouncing back to my PC which I expected would sort the issue out, only to read that these adaptors/cables if used in the orientation i planned literally act just like an HDMI cable and also carry audio.

I am wondering is pulse audio is the problem and when i switch inputs and goes back, it briefly sees the HDMI as an audio option and then switches back to USB, but has been messed up by the switch. What I’m hoping is that it is this and that there is a way to entirely disable audio from the graphics card so changing inputs on my monitor or HDMI switch will never cut out out interfere with my USB DAC. I have 3 USB DACS and have tested all of them so I know these are not at fault either.

Any advice?