DIAGNOSIS Air is trapped in the internal water tube or the heating chamber, usually because the reservoir wasn't filled correctly or scale is blocking smooth water flow.
FIX Fill the water reservoir to the MAX line and run a full brew cycle with just water (no coffee filter). This should purge the air lock. Do this twice if necessary.
IF THAT DIDN'T WORK Mineral scale has likely built up in the narrow inlet tube inside the machine. A thorough descaling (vinegar cycle, let sit for 30 minutes, then flush) is required to dissolve it.
HEADS UP A slight gurgle is normal as the last water is drawn from the reservoir. A loud, shaking rattle is not and indicates an obstruction or air problem.