Hello, when I do short cardio sprints of 30 seconds each at the gym, which bring my heart rate up to around 145-155 bpm for a few seconds, by the 5th or 6th repetition I sometimes feel a "gurgling/bubbly/vibrating' sensation in my heart area. This only happens when I am doing cardio workouts. I am concerned that this might be a sign of something serious with my heart. I am 51, female, reasonably fit, appropriate weight for my age and height. What might be causing this?