DIAGNOSIS The centrifugal brewing unit's bearings are running dry or the unit is obstructed. The motor is spinning but the pump can't move water properly, causing cavitation and noise.
FIX Run a cleaning cycle immediately. With the machine on and empty, press the button three times within two seconds. The light will pulse white. It will run a long water-only cycle to flush the system. Let it complete. This often clears minor scale or coffee particle blockages.
IF THAT DIDN'T WORK Significant limescale buildup has likely seized the bearings of the spinning brew unit. You need to descale. Use only the Nespresso descaling kit. Follow precisely, as the Pop's descaling mode is activated by a specific button sequence (hold button 7 seconds).
HEADS UP Never run vinegar or citric acid through this machine. It will damage the seals and sensors. Only use the official descaling solution. If the grinding noise persists after descaling, the brewing unit motor assembly is failing and needs replacement.