DIAGNOSIS This is almost always a clog in the small internal tube that feeds hot water from the heating chamber up to the spray head. Mineral scale or coffee sediment is the culprit.
FIX Descale the machine. Fill the reservoir with a 50/50 mix of white vinegar and water. Run a full brew cycle without coffee. Let the vinegar solution sit in the carafe for 15 minutes, then pour it back into the reservoir and run a second cycle. Finish with 2-3 clear water brew cycles to rinse.
IF THAT DIDN'T WORK There could be an air lock in the heating system. Try this: with the machine cold and empty, pour a full reservoir of cool water through the machine (run a cycle) before making coffee. This can sometimes clear a vapor bubble causing the noise.
HEADS UP The noise itself isn't usually a sign of imminent failure, but ignoring it will accelerate scale buildup. If you have hard water, descale monthly.