DIAGNOSIS The rotary disc inside the milk frother is either clogged with dried milk residue or has become dislodged. The screech is metal-on-plastic friction.
FIX Detach the frother from its base. Fully disassemble it: remove the lid, the frothing sleeve, and the rotary disc. Wash all parts in warm, soapy water. Use a small brush (a clean toothbrush works) to scrub the holes in the rotary disc and the interior of the frothing sleeve. Reassemble, ensuring the rotary disc clicks securely into place on the pin.
IF THAT DIDN'T WORK The internal spindle that spins the disc may be jammed. Run a cleaning cycle: fill the frother with warm water up to the max line, add two drops of dish soap. Attach it to the machine and run the froth cleaning function (press the milk button for 3 seconds). Repeat with clean water only.
HEADS UP Always rinse the frother immediately after use. Never immerse the base unit where it attaches to the machine in water.