There are a few reasons your cat water fountain might be loud. It could be that the pump is dirty or damaged, causing it to work harder and make more noise. The water level in the fountain could also be low, which would cause the pump to run dry and make a loud grinding noise. Finally, the flow rate of the water could be set too high, which would create a lot of turbulence and splashing. If you're not sure what's causing the noise, take your fountain to a qualified technician for diagnosis and repair.