The pump circulates water at high pressure inside your espresso machine. If it's faulty or clogged with limescale, the water won't flow properly. When this happens, you'll hear a muffled or unusual, very loud sound, then the water will start coming out in small drops or not at all.
One thing you can try is descaling the machine and then testing it again.
If the pump is defective, you will need to replace it.