DIAGNOSIS Air trapped in the internal heating tube or a low water level in the reservoir. The pump is pulling air, causing cavitation.
FIX For the single-serve side, ensure the reservoir is filled above the MIN line. For the 12-cup side, always pour in at least 2 cups of water, even for a single cup. Run a water-only brew cycle (no coffee) on the 12-cup side with a full carafe in place. This often purges the air pocket.
IF THAT DIDN'T WORK Mineral scale has built up on the heating element, causing localized overheating and steam bubbles. Descale the entire unit. Fill reservoir with descaling solution. Run multiple brew cycles through both sides until reservoir is empty.
HEADS UP Persistent loud grinding or screeching is the pump failing. Unplug the unit immediately. Do not attempt to run it further. The pump is not user-serviceable.