Very good questions.
The sensation you are feeling may be your heart. Typically we cannot hear the valves of our heart open or close, or the blood move through them to the different chambers. However, there are exceptions to this. Those with replaced heart valves can feel it. Also, sometimes, those with certain valve abnormalities which cause murmurs (murmurs are caused by abnormal valve function which can cause blood to be forced through a smaller opening than usual or going backward through a valve when it should only go forward) can hear the murmur caused by the blood moving abnormally through the valve.
You state that your PCP said you had a slight murmur years ago. It could be that the murmur is very benign and not at all related to your symptoms; however, it is possible that the murmur could be progressing and be the cause of your symptoms and possibly significant enough that it requires evaluation.
Murmur evaluation usually is performed with an EKG and an Echocardiogram (ultrasound of the heart). These are relatively quick tests (EKG 5 minutes, Echo 1 hour) that do not rely on radiation, are not excessively expensive, are done routinely, and can give excellent information. Considering you are having the symptoms you state, I think this would be a reasonable place to start. If something is noted as abnormal on Echo (valve abnormality) then that can be further evaluated if necessary, or routinely followed for progression.
Thanks very much for the good information.
Just a slight correction: by "popping" and "clicking", I only meant the SOUND I hear (matching my wrist pulse) when reclining; I do not actually FEEL any sensations in my chest.
It merely sounds like I swallowed a clock!!
I thought perhaps with my scoliosis (38-degree curve), when I lie down it's maybe compressing something that causes the pulse-linked clicking sound.
Thanks again for your time,
P.S. The lack of muscle stamina developed slowly over the past 10 years; before that I walked 2 miles almost daily, very fast and effortlessly, but I declined slowly and now walking is a big struggle--but NOT because of shortness of breath, I simply must stop every half block or so and wait a minute before going on slowly due to muscle/hip fatigue. (No hip arthritis per x-rays.)
After some more looking around, I see that other people have had this same (or similar) oddity and never had anything show up on their tests. So I'll just chalk it up to one more unexplainable thing and leave it at that!
I have very, very mild asthma, and there are times when I lie down, particularly if my head is lower than the rest of my body, when I hear a little clicking in time with my pulse. I have no other symptoms, so I figure it's "just one of those things."