I had that issue with both of my Sony Vaios back in the day, they had very bad cooling fans. Sent them to be repaired a total of 3 times.
So if your is still in warranty I’d suggest you do that.
If not then go to a professional cleaning service, because if you’ve already tried the duster and it didn’t work it probably means the dust is far too much lodged into your fan, preventing it from properly cooling and making that noise.
You can also install something like Aida64 and have the CPU / GPU / HDD temp. displayed in your try at all times, you will notice the noise starts to become louder as those components start to heat up (ie: when you are working, and not when you’re idle, as you’ve mentioned).
fan is working at 100% + there is probably some dust there. Open it, used a compressed air can to clean it up and maybe while you’re at it change the thermal paste on the cpu/gpu and maybe the pads on the other components (if it has them)
According to the ancient Mayans, whenever a laptop makes such noise it means that the world is going to end soon.
As the end times will be coming close, your laptop will start making more noise and one day it will just vanish in thin air, poof!
You, my friend, are the chosen one, the messiah to spread this message.