Hi there -- The few people I've encountered who've had ruptured chordae tendonae weren't aware anything specific had happened. They did all experience symptoms, ranging from shortness of breath to lightheadedness to simple fatigue, but none were aware something had mechanically gone haywire. I also find this surprising, or at least I did the first time. It usually comes apart at the far end, where connnected to the papillary muscle, so flailing is definitely the right descriptor, but since the heart muscle has minimal sensory receptors (surprised? Me too) it may not "feel" this even though there is something "loose" in there.
Hope this is helpful. Good luck to you and please follow up with us here as necessary.
John Kenyon, EMT, CCT
Non-invasive cardiology tech, Emergency and Critical Care technician, Critical Incident Stress Mgmt. specialist