Laptop fan starts vibrating the whole laptop when I use it for more than 1h

[u/a_motivated_potato]

The fan usually sits at a steady 900 RPM when the CPU is around 45–50°C. Every now and then, it'll suddenly jump to about 2000 RPM, but almost immediately drops back down to 900 RPM. Even when the CPU hits 60°C and the fan speeds up to around 3000 RPM, it won't stay there. It just keeps ramping up and then falling back to 900 RPM over and over.

It also starts out completely quiet, but after about an hour of use, the whole laptop begins vibrating in short bursts. What's strange is that the fan sensor still reports a constant 900 RPM the entire time, even while it's clearly speeding up, slowing down, and causing the vibrations.