The pump circulates water at high pressure inside your espresso machine. If it is defective or covered in limescale, the water will not be able to flow properly. When this happens, you will hear a muffled or unusual sound, very loud, then the water will begin to come out in small drops or even not come out at all.
One thing you can try is to descale the machine and then test it again.
If the pump proves defective, it must be replaced.