Noisy fans are an indication that they are not working properly which means that heat may be building up inside the computer - a potentially damaging scenario.
Follow these steps to determine why one or more of these fans may be making loud or strange noises.
Clean the CPU fan.
Clean the power supply fan.
If the noise persists and can be narrowed down to the CPU fan, replace the CPU fan.
If the noise persists and can be narrowed down to the fan inside of the power supply, replace the power supply.
Note: Due to safety concerns, it is never recommended to open the power supply and replace only the fan. I always recommend that the entire power supply be replaced instead.
Many fans in perfect working order will simply get louder as the system gets hotter. In this case, it may be necessary to move your PC to a cooler area.
If moving the PC is not an option, you may need to invest in more powerful fans or in a more advanced cooling system like liquid cooling.