DIAGNOSIS That's the sound of the pump trying and failing to draw water. The most likely culprit is a clog in the internal needle that pierces the carafe-style pod (the K-Duo Plus has two separate needles—one for K-Cups, one for the carafe pod).
FIX Turn the machine off and unplug it. Lift the carafe lid and look at the ceiling of the compartment. You'll see a plastic housing with a pointed needle. Use a straightened paperclip or the tool that came with the machine to gently insert into the needle's opening and clear any compacted coffee grounds. Then run multiple water-only carafe cycles (without a pod) to flush it.
IF THAT DIDN'T WORK The water reservoir might not be seated perfectly, breaking the siphon. Remove it, check the rubber float on the bottom for debris, and firmly re-seat it until you hear a distinct click on both sides.
HEADS UP This clog almost always happens because people remove a wet carafe pod immediately after brewing. Always let the pod drain for 30 seconds before removing it, or grounds get sucked back up into the needle.