I have one. I can hear it sometimes at night, rather than kerdonk, kerdonk it goes 'awhoosh, awhoosh'. My father used to get upset if I was puffed when I got to the top of our rather large staircase, but I had no idea why, they were a bit worried but never told me. Of course, when they did, I had no idea what they were talking about anyway. Apart from a little careful monitoring for my pregnancies and a more thorough checkup for a mortgage medical, it has never bothered me, but I do get awkward palpitations at times. Slightly uncomfortable, that's all.