A colleague of mine recently got a MacBook Pro, whenever they plug in an external monitor the fans are pretty much fully on when doing not much at all. They now loathe using it. These computers are so expensive. They seem to have a ton of issues with ridiculous repair costs. What's going on? The keyboard issue from before is another example. Clearly they have the profit margin to ensure their products work well. And…
What actually did the trick - opening the case up, flushing the dust from both fans using compressed air, and replacing the thermal paste on the CPU & GPU. My MacBook is totally silent now.
I'm sure there is more than one possible root cause, but I feel this solution is not nearly as well known as it should be. Laptops generally run hot, and MacBooks have great fans, but they do trap dust. So there are a ton of people running around with old MacBooks which are being absolutely crippled by throttling. I ran some rough benchmarks and performance has approximately doubled. It's super noticeable.
IMO Apple is incentivised to do a poor job in highlighting this (and say, setting up a service program), because when your laptop starts to feel slow after a few years, people tend to feel like maybe it's time to buy a new laptop...
[1] https://www.gearslutz.com/board/music-computers/1322323-macb...
[2] https://forums.macrumors.com/threads/2017-15-mbp-fans-run-w-...