New 4K TV and some NUC HTPC issues (I think)

Apologies for the post length. Just keen to get as much info out as possible to help with the fix.

1. Just received my new 4K Samsung QLED 75" telly.

2. My NUC is getting on a bit - Model is D54250WYK. It's an I5-4250 CPU @ 1.3 GHz. 8 GB Ram

3. My AV Receiver does upto 1080p and is Denon AVR1713

4. Running Win 10 on NUC

After lots of frustration about 5 years ago ( look back through geekzone) getting the previous TV setup, I had to bypass the NUC HDMI to receiver connection as the old TV which was Ultra High Def kept convincing the NUC to switch resolution and ouput higher than 1080p. The AV Receiver could only display 1080p so kept giving out an error. Eventually fixed it by plugging the NUC directly into the TV on HDMI 2 and then plugging an optical audio cable from the TV back to the receiver. So watch the NUC on HDMI 2 and listen using the AUX on the amp. Sweet. Worked for 5 years until TV started to die.

So today I took the opportunity to re-arrange all the AV gear, unplugged it all, dusted the shelves and put it back together - same as above but with new TV.

Things i noticed

1. NUC now set resolution at 3840 × 2160 ( recommended ). While the desktop looked amazing and most things seemed to run including Kodi, when i maxed out YouTube it became choppy / almost dropping frames. Watchable but you could tell something was off. The issue wasn't there when youtube was playing in a smaller window but when maxed it returned. According to online information, solution was to drop the resolution on the NUC back to 1080p - that seemed to do the trick. Youtube back to smooth full screens.

2. I have an intermittent / very low volume 'static sounding' crack coming through the speakers. Last less than a second. Every 2 or 3 minutes. Happens when windows just sitting on desktop as well as playing stuff through Kodi and Youtube. I haven't heard it when using SKY input so 'assumed' it's NUC issues. I have just updated the audio drivers as well but no difference. Happens also after 'restarting' the NUC - black screen while booting then about 2 or 3 static sounding cracks then windows theme tune plays through the speakers ( albeit missing the first couple of seconds) and the login page appears. Could it be because i pulled stuff apart and put it back together or is it more likely NUC hardware / drivers ? I remember having this several years back but can't remember how it was fixed / if i did something.

Questions

1. I assume the NUC which was made late 2014 according to the bios is struggling to run 4k resolution / youtube. If so any recommendations on what NUC model specs would ?

2. Re the static sounding crack through the speakers - I'm lost if truth be told. Was going to break and remake connections tomorrow, change AC plugs around, make sure speaker cables are well away from power etc.

3. I've just found and updated new realtek audio drivers for ' Legacy " NUC's running win 10. No difference.

4. A new AV receiver that does 4k should allow me to plug everything including the NUC into it using HDMI. I'm wondering if that might help the situation / remove the bodge using the optical audio cable.

Thanks for reading and any suggestions to try. Happy to answer any questions.

Cheers

Ged