My old 980Ti started doing this around the 4 year mark of owning it. It would do it randomly as the computer was just sitting idle, while playing a game or even just watching a video. It didn't do it all the time, but enough to make it a concern (for a while, I eventually just learned to ignore it and live with it).
I removed the OC (which was pretty hefty since that card could OC like crazy), adjusted voltages, adjusted the fan curve, removed the fan curve and so on. None of them changed the behaviour. Last step I did was I pulled the card apart, repasted, replaced the thermal pads and completely cleaned it to the point it looked almost unused. Sadly, the problem persisted. I used the card for another 2-2.5 years without any issues. I even sold the card (was the height of the crypto boom) and explained the odd behaviour it had to the buyer so they weren't taken by surprise and they didn't care. Hopefully it lasted and ran for a few more years for the buyer.
Yeah, you need to check things out, even though you can see into the case (based on the picture you provided), you should still pop the side cover off and just get a good peek at the GPU when you start up a game that causes this issue. Make sure all the fans on it are spinning.
I did have one fan of the three on my current 3080Ti decide not to spin one day. My system was sounding very loud because only two fans were spinning on the GPU and they were working harder to keep it cool. I only noticed this once I removed the cover on my case. I saw two fans spinning and one just sitting there doing nothing. I gave it a little nudge with my finger and the fan started spinning again. I haven't had the issue since. I don't know why it stopped, the card isn't dirty. Just an odd situation.
