DIAGNOSIS: That's the pump running dry or struggling to pull water from the external reservoir into the internal heating tank. Usually means an air lock in the system.
FIX: First, check that the external reservoir is seated all the way down—sometimes it looks in place but isn't clicked fully. If it is, unplug the unit. Remove the reservoir and pour the water out. Plug the machine back in without the reservoir. Let it sit powered on for 2 minutes (you might hear the pump try and fail). Then, reinstall a fully filled reservoir. The pump should prime itself quietly.
IF THAT DIDN'T WORK: The one-way valve in the water line might be stuck. You can try a manual prime: with the unit off and reservoir full, hold the 'hot water' button down for 15 seconds, then release.
HEADS UP: Letting the external reservoir run completely dry is the main cause of this. Try to refill it before it hits the empty line to avoid the air lock.