DIAGNOSIS That choking sound is the dock's motor straining because the airflow path is blocked. It's not the main bin; it's a clog in the internal tubing or the robot's dust port.
FIX 1) Unplug the dock. 2) Remove the dust bag and look inside the empty chamber. Use a flashlight to check the opening where the bag sits—you might see a clump of dust and debris. Carefully clear it with a long, thin tool (a straightened coat hanger works). 3) Now, check the robot's side. Clean the rubber seal around its dust port. 4) Most importantly, take out the robot's main dustbin and look at the small, rectangular hole on the back of it. Clear any compacted dust from that opening and the filter inside the bin.
IF THAT DIDN'T WORK The clog could be deeper in the hose between the dock's opening and the bag chamber. Detach the hose from the dock (if your model allows) and blow through it or use a vacuum to pull debris out from the other end.
HEADS UP Never use compressed air to blast a clog deeper into the dock—you could damage the internal motor. Always work from the bag chamber back towards the opening. And empty the robot's bin more frequently than the dock's bag; a overfull robot bin is the number one cause of these clogs.