When your computer’s fan seems to be louder and come on more often than usual, it probably means it’s a good idea to open it up and blow the dust out.
Our main living room HTPC, an Intel NUC6i7KYK, was seeming really loud, even when not under load. Looking back, it was clearly one of those things where it must have been getting steadily worse over time, but so gradually that I hadn’t actually noticed just how much louder it was and how much more often the fan was at high speeds than when we first got it. Opening it up and blowing the dust out (and it really didn’t seem like that much dust at the time) substantially improved things.
The CPU was running around 70°C even with no load, and pushing 80°C with the fan on full speed when under load. After blowing out the dust, it’s now around 50°C or even less, and even under load I haven’t seen it over 60°C and the fan is still quiet enough to not really be noticeable. It’s just such a small computer that even just a bit of dust can make a big impact. I clearly should have been monitoring it more carefully, I just hadn’t even thought to do so until it had gotten so bad that we noticed a problem and I started taking measurements.