CPU Fan being loud and slowing down, then squeeking

Hello, my CPU fan runs normally and quite quietly for most of the time. However, after spending some hours on the computer, the fan spins at 1900RPM and then becomes quite loud, shaking the entire machine. In order to make it go quiet I must hold the case and press it down hardly at the top. The computer is only slightly quieter when pressing down. The temperature of the CPU is at around 40°C when it occurs. When the loud noise occurs and trembling, the CPU fan speed goes down to 1800RPM. However, after some more 10 minutes of usage and no change in CPU temparature, the fan becomes extremely loud (screeches like car brakes scratch across the wheel plate, very high pitch) and the fan speed drops down to 1200RPM really quickly, and probably goes down even more but I'm afraid to run it beyond this point. After this happens the noise will reoccur after turning on the PC and only after about 10 hours I can run it without the noise. However, my CPU speed almost never exceeds 45°C.
It's an ASUS motherboard, and it has the Q Cpu Fan something enabled in the BIOS settings.

CPU: AMD A10-7800
GPU: None, integrated in the CPU
RAM: 8GB
Stock fan, stock motherboard